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Automotive engine lubricants
(2019)
In 2003, ASTM Manual 37, Fuels and Lubricants Handbook: Technology, Properties, Performance and Testing, featured a chapter discussing automotive lubricants, including engine oils, automatic transmission fluids, manual transmission fluids, gear lubricants, and greases. This chapter, by Schwartz, Tung, and McMillan, surveyed all of these classes of lubricants, up to its publication in 2003.
More recently, the period between 2003 and 2010 has been addressed as part of a book copublished by ASTM and SAE International (coeditors Simon Tung and George Totten), where Fox surveyed the development of engine oil specifications that emerged during those years.
This chapter surveys the evolution of engine oil design and testing since 2002 and concludes with insights into future directions offered by recent tribological research. Our objective is to provide a reader new to the field with an understanding of the following:
- Engine oil composition and formulation
- North American engine oil specification development, and an outline of emergent European specifications
- How emergent specifications and legislative requirements are linked to the introduction of new engine hardware
- How tribological innovation can contribute to future enhancements in engine efficiency
In parallel, we will provide several tables comparing groups of contemporaneous specifications.
Additive Fertigungsverfahren gewinnen aufgrund der schnellen, flexiblen und kosten-günstigen Fertigung von Bauteilen zunehmend an Bedeutung. Das Laserpulverauf-tragschweißen (LPA) wurde anfangs hauptsächlich als Beschichtungsverfahren ein-gesetzt. Diese Technologie bewerkstelligt aber auch das Reparieren von verschlisse-nen Bauteilen, sodass diese zeitsparend und ressourcenschonend erneuert werden können. Die hohe Aufbaurate, die flexible Pulverzusammensetzung sowie die hohe Endkonturnähe ermöglichen heutzutage die Entwicklung und additive Fertigung von neuen Materialien, wie zum Beispiel Nickel-Titan-Formgedächtnislegierungen.
The determination of mechanical properties of welded Steel structures such as strength or ductility is a subject of high interest for the majority of Companies in the area of metal Processing. The material Parameters can be obtained by performing the tensile test on the samples made from a part of a component. In some cases, it is highly expensive to produce the tensile specimens especially from the weld metal, which contains different type of microstructure such as weld seam or heat affected zone in an extremely small area. Therefore, a method is described in this paper to determine the material Parameters of high strength Steel structures and welded joints locally and without any additional effort to perform the tensile test. In this method, instrumented indentation technique (IIT), an indenter is pushed on the flat surface of a specimen in a certain period of time and simultaneously the applied force and the corresponding indentation path are measured. The data related to the force-indentation diagram is given as input to an artificial neural network (ANN) to obtain the material Parameters. The ANN can be trained by generating the large qualitative data sets with numerical Simulation of the IIT procedure. The Simulation must be run several times with the different material model parameter sets to generate the numerous
force-indentation diagrams as the inputs of ANN. Then, the trained ANN is validated by performing the IIT on the welded joints and comparing the obtained material Parameters from ANN with the tensile test.
Consequently, the mechanical properties of welded joints can be determined by performing the IIT and evaluating the resulting data by the ANN.
To increase the competitiveness of jacket substructures compared to monopiles a changeover from an individual towards a serial jacket production based on automated manufactured tubular joints combined with standardized pipes has to be achieved. Therefore, this paper addresses fatigue tests of automatically welded tubular X-joints focusing on the location of the technical fatigue crack. The detected location of the technical crack is compared to numerical investigations predicting the most fatigue prone notch considering the structural stress approach as well as the notch stress approach. Besides, the welding process of the automated manufactured tubular X-joints is presented.
Solidification cracking of metals is a well-researched topic in the field of welding science. A material’s susceptibility to solidification cracking can be tested using numerous different specialized test procedures, one of which is the Modified Varestraint-/Transvarestraint test (MVT). It was developed at BAM in 1982 and is internationally standardised. Over the decades, this test has been extensively used to characterise the solidification cracking resistance of many different materials.
The present study was conducted to further investigate the influences of the standardised MVT testing parameters, as well as the characteristics of evaluation methods on the results. Several different high alloyed martensitic LTT (low transformation temperature) filler materials, CrNi and CrMn type, were used. In previous pilot studies, these alloys have shown a rather distinctive solidification cracking behaviour (Fig. 1). During testing, energy input per unit length and bending speed were varied (especially the latter is usually kept at standard values), in addition to the most commonly altered factor - total deformation.
First, the effects of different process parameter sets on the solidification cracking response were measured using the standard approach - microscopic analysis of the specimen surface. It was found that not all parameter changes had the expected outcome. For the Cr8Ni6 and Cr11Mn5 filler materials, influences of energy input per unit length and welding speed were in direct opposition.
In order to investigate those apparent contradictions, µCT scans of MVT specimens were made. The results consistently show sub surface cracking, to significant, yet varying extents. Different primary solidification types were found using WDX-analysis, an aspect that is believed to be the main difference between the CrNi- and CrMn-type materials and their cracking characteristics.
Results show that when it comes to testing of modern high-performance alloys, one set of standard MVT testing parameters might not be equally suitable for all materials. Also, to properly accommodate different solidification types, sub-surface cracking has to be taken into account.
Als Ausgangspunkt für i-TRIBOMAT schließen vier renommierte Institutionen in Europa, darunter die BAM, ihre Test- und Analytik-Methoden zusammen. Es entsteht ein Testfeld aus weit über 100 Tribometern mit den entsprechenden, analytischen Kapazitäten. Alle Ergebnisse fließen in eine harmonisierte Datenbank, die tribologische Erkenntnisse mit Materialeigenschaften ebenso zugänglich macht, wie Hinweise auf ökologische und gesundheitliche Verträglichkeiten. Diese Datenbank bildet zusätzlich das Rückgrat einer Vielzahl von Modellierungsmethoden, die die Entwicklungszeiten von der Idee bis zum marktreifen Produkt deutlich verkürzen sollen. Alle diese Möglichkeiten sollen über eine europaweit agierende, zentrale Kontaktstelle zur Verfügung gestellt werden.
Im Beitrag sind die Vor- und Nachteile beim Einsatz von Ultraschallunterstützung für das Zerspanen von harten und spröden Werkstoffen zusammengefasst. Des Weiteren werden aktuelle und zukünftige Analysen zum Thema ultraschallunterstütztes Zerspanen mit geometrisch bestimmter Schneide schwer zerspanbarer Materialien vorgestellt.
Over the last years, the weight of modern car bodies has risen significantly due to the increasing customers’ demand for comfort and safety equipment. However, this ongoing trend leads to an increasing fuel consumption and thus to higher carbon dioxide emissions. In order to counteract these problems, hot stamping has been established in the automotive industry as a key technolo-gy for lightweight construction, regarding the manufacturing of safety-relevant car body compo-nents. Hot stamped parts are commonly made out of boron-manganese steel 22MnB5, which is initially austenized and subsequently formed and quenched in one process step. As a result, geo-metrical complex structures with an ultimate tensile strength of 1500 MPa are generated. The surfaces of the workpieces are coated with an Al-Si layer to avoid oxide scale formation and to ensure corrosion protection. However, the coating system leads to an increased adhesive wear on the tool surface due to the high thermo-mechanical tool stresses. Therefore, a time and cost con-suming rework of the hot stamping tools is required. The aim of this study is to increase the tribological performance of hot stamping tools by using a laser implantation process. This tech-nique allows the ma-nufacturing of separated, elevated and dome-shaped microstructures on the tool surface in consequence of a localized dispersing of hard ceramic particles by pulsed laser radiation. The generated surface features offer great potential for reducing the tribological load, due to their high hardness and wear resistance. For this purpose, the friction coefficient of un-modified and laser implanted tool surfaces were examined and compared by using a modified pin-on-disk test. In addition, the surfaces were analyzed by optical measurements in order to quantify the amount of wear.
Wirtschaftliche und ökologische Aspekte führten in den letzten Jahren zu deutlich gesteigerten Anforderungen an die Effizienz und die Flexibilität petrochemischer Anlagen. Die heutzutage geforderten Prozesstemperaturen und -drücke lassen sich nur durch den Einsatz neuer warmfester Stahlgüten erreichen. Der mit Vanadium modifizierte Stahl 13CrMoV9-10 weist eine bessere Kriech- und Druckwasserstoffbeständigkeit auf und wird seit Mitte der 90er Jahre im petrochemischen Reaktorbau eingesetzt. Aufgrund der niedrigen Zähigkeit und hohen Festigkeit des Schweißgutes im geschweißten, nicht spannungsarm geglühten Zustand sowie einer erhöhten Sensitivität gegenüber Spannungsrelaxationsrissen bedarf dieser Stahl allerdings einer äußerst sorgfältigen schweißtechnischen Verarbeitung. Bisherige Untersuchungen zur Rissentstehung in warmfesten Stählen konzentrierten sich vorrangig auf thermische und metallurgische Einflussfaktoren, bieten jedoch nur wenige Erkenntnisse zum Einfluss des Schweißprozesses auf die Rissbildung beim Spannungsarmglühen unter Berücksichtigung realitätsnaher Fertigungsbedingungen.
Im ersten Teil wurde zunächst der Einfluss der Wärmeführung auf die mechanischen Eigenschaften anhand von frei schrumpfenden Laborproben untersucht. Während sich die Wärmeführung wesentlich auf die Schweißnahtstruktur auswirkte, war ein signifikanter Effekt auf die mechanischen Eigenschaften nicht nachweisbar. Auch traten infolge der Wärmenachbehandlung der frei schrumpfend geschweißten Proben keine Spannungsrelaxationsrisse auf.
Der zweite Teil umfasste die realitätsnahe Abbildung der Fertigungsbedingungen im petrochemischen Reaktorbau. Zu diesem Zweck wurden die konstruktiven Randbedingungen während des Vorwärmens, Schweißens, des Wasserstoffarmglühens und der abschließenden Wärmenachbehandlung realitätsnah in einer speziellen 3-D-Prüfanlage zur Simulation von Bauteilschweißungen abgebildet. Unter konstruktiver Schrumpfbehinderung gelang der Nachweis der unterschiedlichen Wirkung von Vorwärm- / Zwischenlagentemperatur und Streckenenergie auf die resultierenden Kräfte, Momente und Spannungen. Die Gesamtreaktionsspannung nach dem Schweißen wurde innerhalb des untersuchten Parameterfeldes maßgeblich durch die Streckenenergie beeinflusst.
Ein möglichst geringer Gesamtwärmeeintrag hat die niedrigste Bauteilbeanspruchung zur Folge. Während der Wärmenachbehandlung kam es in allen Versuchen zu Spannungsrelaxationsrissen. Deren kumulierte Länge korrelierte mit den ermittelten schweißbedingten Reaktionsspannungen.
Die Detektion der Risse während der Wärmenachbehandlung im Bauteilversuch erfolgte erstmalig in-situ mittels Schallemissionsanalyse. Die Rissinitiierung fand nachweislich im Temperaturbereich von 300 °C bis 500 °C statt. Die Reaktionsspannungen nach der Wärmenachbehandlung lagen unabhängig von der Ausgangsbelastung auf einem vergleichbaren Niveau. Die Zähigkeit der Schweißverbindung nahm klar gegenüber den frei schrumpfend geschweißten Proben ab.
Mittels REM- und TEM-Analysen an vergleichbaren belasteten und unbelasteten Proben wurde eine beschleunigte Alterung, durch das frühzeitige Ausscheiden von Sonderkarbiden während der Wärmenachbehandlung unter definierter Einspannung (d. h. unter Belastung), nachgewiesen. Die verstärkte Korngrenzensegregation korrelierte mit der signifikanten Abnahme der Kerbschlagarbeit der unter Einspannung geschweißten Proben.
Plasma-Transferred-Arc (PTA) welding is a process that enables high deposition rates, but also causes increased thermal load on the component. Laser metal deposition (LMD) welding, on the other hand, reaches a high level of precision and thus achieves comparatively low deposition rates, which can lead to high processing costs. Combining laser and arc energy aims to exploit the respective advantages of both technologies. In this study, a novel approach of this process combination is presented using a PTA system and a 2 kW disk laser. The energy sources are combined in a common process zone as a high-speed plasma laser cladding technology (HPLC), which achieves process speeds of 10 m/min at deposition rates of 6.6 kg/h and an energy per unit length of 39 J/mm.
Resistance spot welding (RSW) is widely used in the automotive industry
as the main joining method. Generally, an automotive body contains
around 2000 to 5000 spot welds. Therefore, it is of decisive importance to characterize the mechanical properties of these areas for the further optimization and improvement of an automotive body structure. The present paper aims to introduce a novel method to investigate the mechanical properties and microstructure of the resistance spot weldment of DP1000 sheet steel. In this method, the microstructure of RSW of two sheets was reproduced on one sheet and on a bigger area by changing of the welding parameters, e. g. welding current, welding time, electrode force and type. Then, tensile tests in combination with digital Image correlation (DIC) measurement were performed on the notched tensile specimens to determine the mechanical properties of the weld metal.
The notch must be made on the welded tensile specimen to force the fracture and elongation on the weld metal, enabling the characterization of its properties. Additionally, the parameters of a nonlinear isotropic material model can be obtained and verified by the simulation of the tensile specimens. The parameters obtained show that the strength of DP1000 steel and the velocity of dislocations for reaching the Maximum value of strain hardening, are significantly increased after RSW. The effect of sample geometry and microstructural inhomogeneity of the welded joint on the constitutive property of the weld metal are presented and discussed.
The development of hydrogen technologies is a key strategy to reduce greenhouse gas emission worldwide. Power-to-Gas is a challenging solution, in which hydrogen and methane can be used in mobility, industry, heat supply and electricity generation applications. This presentation deals with the tribological behaviour of polymer materials in hydrogen and methane, both in gas and in liquefied form.
The development of hydrogen technologies is a key strategy to reduce greenhouse gas emission worldwide. Power-to-Gas is a challenging solution, in which hydrogen and methane can be used in mobility, industry, heat supply and electricity generation applications. This presentation deals with the tribological behaviour of polymer materials in hydrogen and methane, both in gas and in liquefied form.
Der Implant-Test nach ISO 17642-3 gehört zur Gruppe der fremdbeanspruchten Kaltrisstests, bei dem eine äußere Beanspruchung auf einen zylindrischen Stab (Implant-Probe) definiert aufgebracht wird. Mit dieser Kaltrissprüfung wurde vergleichende Analysen zur Kaltrissempfindlichkeit der modernen hochfesten Feinkornbaustähle S960MC und S960QL durchgeführt.
Heute werden in Stahlkonstruktionen zunehmend hochfeste Baustähle (Streckgrenze ≥ 960 MPa) eingesetzt. Daher sind auch in Schweißnähten solche hohen Festigkeiten gefordert. Jedoch können hohe Eigenspannungen aufgrund geringerer plastischer Dehnungsreserven in diesen Schweißnähten die Sicherheit der Bauteile vermindern. Diese sind insbesondere im Zusammenhang mit Wasserstoff und Härtegefügen risskritisch. Niedrige Bauteilbeanspruchungen und Kosteneinsparungen lassen sich heutzutage mit modernen fokussierten Lichtbogenvarianten und engeren Nahtspalten erreichen. Die vorliegende Studie zeigt allerdings, dass diese modernen Schweißprozesse und die angepasste Nahtgeometrie höhere Wasserstoffkonzentrationen und signifikant gesteigerte oberflächennahe Zugeigenspannungsniveaus bedingen. Anhand mehrlagiger Schweißversuche an S960QL unter Variation von Wärmeführung, Lichtbogenvariante und Nahtöffnungswinkel wurden die Effekte und Wechselwirkungen unter Zuhilfenahme von Eigenspannungsanalytik mittels Röntgendiffraktometrie und Wasserstoffanalytik mittels Trägergasheißextraktion systematisch untersucht. Das Schweißgefüge führte unter kritischer mechanischer Beanspruchung und erhöhtem Gehalt an diffusiblem Wasserstoff insbesondere bei geringem Nahtöffnungswinkel und fokussiertem Lichtbogen zu einer deutlichen Ausbildung von Mikrorissen. Die Mikrorisse führten teils zu einer makroskopischen Ausprägung. Durch Analysen zur Nachwärmung aus der Schweißwärme heraus konnten suffiziente Wärmeführungsparameter für eine effektive Prävention solcher Risse erarbeitet werden. Die vorliegende Arbeit zeigt anhand des Interaktionssystems bei der wasserstoffunterstützten Kaltrissbildung Gefüge-Beanspruchung-Wasserstoff, welche Wärmeführungs- und Schweißparameter sich für die hochfesten Feinkornbaustähle zur Kaltrissvermeidung eignen.
The paper provides an overview on material defects which may serve as fatigue crack initiation sites and can cause final fatigue failure of a component. These comprise nonmetallic inclusions in Steel and aluminum alloys, cavities such as pores, micro-shrinkages and un-welded regions in cast, sinter and additively manufactured alloys, graphite nodules, shrinkages and other items in modular cast iron, regions of defective microstructure, microcracks and secondary notches such as undercuts and surface roughness. Besides their origin, the effect and mechanisms on fatigue crack initiation and propagation are discussed.
The considerations are proceeded and accompanied by a Brief discussion of some Basic aspects such as the stages of crack propagation along their length scale, the overcoming of crack arrest and the question when a secondary notch can be treated as a crack.
Effect of Inter-Layer Dwell Time on Distortion and Residual Stresses of Laser Metal Deposited Wall
(2019)
The laser metal deposition is an advanced manufacturing technology enabling the production of large-sized parts and partially or completely elimination of machining and welding. The process is characterised by non-uniform local heating of the buildup leading to a stress distribution, which may exceed the yield strength of the material and leads to loss of dimensional accuracy. The interlayer dwell time has a strong influence on the temperature field. The effect of the interlayer dwell time on the distortion and the stress distribution during laser metal deposition of a single-pass wall on the edge of 2 mm thick plate was studied experimentally and numerically. The deposited material was IN625 and the substrate material was AISI 316. A decrease of the residual displacement, due to a uniform shrinkage after the deposition of the last layer and a lower level of the residual compressive longitudinal plastic strain, has been observed in the studies without dwell time. The peak increment of the free edge displacement corresponds to the first layer and hence the subsequent layers will be deposited on the already plastically deformed buildup. The tensile residual longitudinal stress near the top of the buildup and transverse stress near the edges of the buildup is higher than yield strength in the studies with dwell time.
Some zinc coated advanced high strength steels (AHSS), under certain manufacturing conditions, are known to be prone to liquid metal embrittlement (LME) during resistance spot welding. LME is an undesired phenomenon, which can cause both surface and internal cracks in a spot weld, potentially influencing its strength. An effort is made to understand influencing factors of LME better, and evaluate geometry-material combinations regarding their LME susceptibility. Manufacturers benefit from such knowledge because it improves the processing security of the materials. The experimental procedure of welding under external load is performed with samples of multiple AHSS classes with strengths up to 1200 MPa, including dual phase, complex phase and TRIP steels. This way, externally applied tensile load values are determined, which cause liquid metal embrittlement in the samples to occur. In the future, finite element simulation of this procedure gives access to in-situ stress and strain values present during LME formation. The visualization improves the process understanding, while a quantification of local stresses and strains allows an assessment of specific welded geometries.
Der Vortrag behandelt in grundsätzlicher weise die Anwendung bruchmechanischer Methoden auf die rechnerische Ermittlung der Wöhlerkurve. Dabei werden die Besonderheiten des Kurzrisswachstums (Notwendigkeit der elastisch-plastischen Beschreibung der zyklischen Rissspitzenbelastung, gradueller Aufbau des Rissschlißphänomens) ebenso diskutiert wie das Problem des Mehrfachrissewachstums und Rissarrest. Die Ausführungen werden an einem Beispiel illustriert.
Among the various welding technologies, resistance spot welding (RSW) and laser beam welding (LBW) play a significant role as joining methods for the automobile industry. The application of RSW and LBW for the automotive body alters the microstructure in the welded areas. It is necessary to identify the mechanical properties of the welded material to be able to make a reliable statement about the material behavior and the strength of welded components. This study develops a method by which to determine the mechanical properties for the weldment of RSW and LBW for two dual phase (DP) steels, DP600 and DP1000, which are commonly used for the automotive bodies. The mechanical properties of the resistance spot weldment were obtained by performing tensile tests on the notched tensile specimen to cause an elongation of the notched and welded area in order to investigate its properties. In order to determine the mechanical properties of the laser beam weldment, indentation tests were performed on the welded material to calculate its force-penetration depth-curve. Inverse numerical simulation was used to simulate the indentation tests to determine and verify the parameters of a nonlinear isotropic material model for the weldment of LBW. Furthermore, using this method, the parameters for the material model of RSW were verified. The material parameters and microstructure of the weldment of RSW and LBW are compared and discussed. The results show that the novel method introduced in this work is a valid approach to determine the mechanical properties of welded high-strength steel structures. In addition, it can be seen that LBW and RSW lead to a reduction in ductility and an increase in the amount of yield and tensile strength of both DP600 and DP1000.
Vermeidung von Schweißimperfektionen im Überlappbereich bei laserstrahlhybridgeschweißten Rundnähten
(2019)
Die Laserstrahl-Hybrid-Schweißtechnologie erweist sich in der schweißtechnischen Fertigung immer mehr als innovative Alternative gegenüber anderen Schweißverfahren. Obwohl das Laserstrahl-Hybridschweißverfahren viele wirtschaftliche Vorteile gegenüber herkömmlichen Schweißverfahren aufweist, wie z.B. große Einschweißtiefe und dadurch eine reduzierte Anzahl von Schweißlagen, geringe thermische Belastung des Grundwerkstoffes aufgrund reduziertem Wärmeeintrag, konnte das Schweißverfahren überwiegend für das Schweißen von Längsnähten demonstriert werden. Eine Vielzahl von Schweißaufgaben, z.B. beim Schweißen von Segmenten von Windkraftanlagen oder dem Orbitalschweißen beim Verlegen von Großrohrleitungen sieht vor, dass die zu schweißenden Bauteile mit einer Rundnaht zusammengefügt werden. Die schweißtechnische Herausforderung ist hier, dass beim Schließen einer Rundnaht mit der Entstehung eines fehlerbehafteten Überlappbereiches zu rechnen ist. Ein zentrales Problem im Überlappbereich einer laserstrahl- sowie laserhybridgeschweißten Rundnaht ist die Bildung von Imperfektionen wie Poren, Rissen sowie die Bildung eines Endkraters, welcher als geometrische Kerbe wirkt. Bisher liegen keine universellen Lösungen zur fehlerfreien Ausführung von geschlossenen Rundnähten beim Laserstrahl-Hybridschweißen vor. Diese Studie befasst sich mit der Entwicklung eines Verfahrens, mit dem die Entstehung von o.g. Schweißimperfektionen vermieden wird. Die Strategie der Prozessführung beim Schließen der Rundnaht sieht hervor, dass ein fehlerfreier Überlappbereich durch die Kontrolle der Erstarrungsbedingungen am Schweißnahtende erreicht werden kann. Die kontrollierte Wärmeführung wird durch eine Anpassung der Parameter von beiden beteiligten Schweißprozessen, dem Laserstrahl- sowie MSG-Schweißprozess realisiert.
Im Rahmen dieser Arbeit wurde eine Serie von Schweißversuchen an 9,5 mm dicken Rohabschnitten aus hochfestem Pipelinestahl X100Q mit Variation der Prozessparameter wie der Laserleistung, der Defokussierung des Laserstrahls sowie der Endkraterfüllzeit im Überlappbereich der Rundnaht durchgeführt. Nachfolgend werden die erzielten Ergebnisse dargestellt und diskutiert.
Renowned institutions in the field of tribology combine their testing and analytical capabilities with experts in materials/process information management technology to provide the respective services in Europe (i-TRIBOMAT). This requires a centralized materials information management system or the standardised capture, consolidation and harmonization of tribological information.
Renowned institutions in the field of tribology combine their testing and analytical capabilities with experts in materials/process information management technology to provide the respective Services in Europe (i-TRIBOMAT). This requires a centralized materials information management system or the standardised capture, consolidation and harmonization of tribological information.
The introduction of electromagnetic stirring to laser beam welding can bring several beneficial effects e.g. element homogenization and grain refinement. However, the underlying physics has not been fully explored due to the absence of quantitative data of heat and mass transfer in the molten pool. In this paper, the influence of electromagnetic stirring on the thermo-fluid flow and element transport in the wire feed laser beam welding is studied numerically and experimentally. A three-dimensional transient heat transfer and fluid flow model coupled with dynamic keyhole, magnetic induction and element Transport is developed for the first time. The results suggest that the Lorentz force produced by an oscillating magnetic field and its induced eddy current shows an important influence on the thermo-fluid flow and the keyhole stability. The melt flow velocity is increased by the electromagnetic stirring at the rear and lower regions of molten pool. The keyhole collapses more frequently at the upper part. The additional Elements from the filler wire are significantly homogenized because of the enhanced forward and downward flow. The model is well verified by fusion line shape, high-speed images of molten pool and measured element distribution. This work provides a deeper understanding of the transport phenomena in the laser beam welding with magnetic field.
The additional element from the filler wire in the laser beam welding is usually distributed inhomogeneously in the final weld due to the high solidification rate of weld pool. It has been found that the electromagnetic stirring produced by an external oscillating magnetic field can enhance the material mixing in the weld pool to achieve a more uniform element distribution. However, the magnetic field has a highly non-linear and multi-coupled interaction with the weld pool behavior, which makes the quantitative explanation of the physical mechanism difficult. In this study, the effect of electromagnetic stirring on the transport phenomena in the wire feed laser beam welding is investigated by a numerical modelling. A 3D transient multi-physical model considering the magnetohydrodynamics, heat transfer, fluid flow, keyhole dynamics and element transport is developed. The multiple reflections and the Fresnel absorption of the laser on the keyhole wall are calculated using the ray tracing method. The numerical results show that a Lorentz force produced by the oscillating magnetic field and its induced eddy current gives significant influence on the transport phenomena in the molten pool. The forward and downward flow is enhanced by the electromagnetic stirring, which homogenizes the distribution of the additional elements from a nickel-based filler wire in a steel weld pool. The numerical results show a good agreement with the high-speed images of the molten pool, the fusion line from the optical micrograph and the element distribution from the energy dispersive X-ray spectroscopy. This work provides a physical base for the electromagnetic-controlled laser beam welding and some guidance for the selection of electromagnetic parameters.
The control of friction and wear is a major concern in many industrial applications. A promising method for a tailored surface modification is the so-called laser implantation technique. This method combines surface texturing and material optimization in one processing step by a localized dispersing of hard ceramic particles using pulsed laser radiation. Wear resistant, protruding micrometric features (implants) with defined geometry can be created in deterministic pattern where needed on highly stressed surfaces, i.e. on forming or cutting tools. However, in order to maintain the implants over the tool’s lifetime, a suitable selection of hard ceramic particles is a prerequisite. They must provide a defect-free Metal Matrix Composite with a high share of homogeneously distributed particles and especially a high implant hardness.
In this study TiN, TiC and TiB2 hard particles were compared as implant materials for the first time. By a systematic variation of the pulse power and pulse duration, their dispersing behavior and influence on the material properties of AISI D2 tool steel was investigated. Although all powder materials had grain sizes smaller than 10 µm, it was possible to disperse them by pulsed laser radiation and to obtain defect-free protruding implants. The highest share of dispersed particles (~64 %) was observed for TiB2. By scanning electron microscopy and energy dispersive X-ray spectroscopy, it was also shown that a significant share of the pre-placed particles was dissolved by the laser beam and precipitated as nanometer sized particles within the matrix during solidification. These in-situ formed particles have a decisive influence on the material properties. While the TiN and TiC implants have shown maximum hardness values of 750 HV1 and 850 HV1, the TiB2 implants have shown the highest hardness values with more than 1600 HV1. By X-ray diffraction, it was possible to ascribe the lower hardness values of TiC and TiN implants to high amounts of retained austenite in the metal matrix. By implanting TiB2, the formation of retained austenite was successfully suppressed due to the in-situ formation of TiC particles, which was proven by electron backscatter diffraction. In conclusion, all the implant materials are basically suitable for laser implantation on AISI D2 tool steel. However, TiB2 has shown the most promising results.
The control of friction and wear is a major concern in many industrial applications. A promising method for a tailored surface modification is the so-called laser implantation technique. This method combines surface texturing and material optimization in one processing step by a localized dispersing of hard ceramic particles using pulsed laser radiation. Wear resistant, protruding micrometric features (implants) with defined geometry can be created in deterministic pattern where needed on highly stressed surfaces, i.e. on forming or cutting tools. However, in order to maintain the implants over the tool’s lifetime, a suitable selection of hard ceramic particles is a prerequisite. They must provide a defect-free Metal Matrix Composite with a high share of homogeneously distributed particles and especially a high implant hardness.
In this study TiN, TiC and TiB2 hard particles were compared as implant materials for the first time. By a systematic variation of the pulse power and pulse duration, their dispersing behavior and influence on the material properties of AISI D2 tool steel was investigated. Although all powder materials had grain sizes smaller than 10 µm, it was possible to disperse them by pulsed laser radiation and to obtain defect-free protruding implants. The highest share of dispersed particles (~64 %) was observed for TiB2. By scanning electron microscopy and energy dispersive X-ray spectroscopy, it was also shown that a significant share of the pre-placed particles was dissolved by the laser beam and precipitated as nanometer sized particles within the matrix during solidification. These in-situ formed particles have a decisive influence on the material properties. While the TiN and TiC implants have shown maximum hardness values of 750 HV1 and 850 HV1, the TiB2 implants have shown the highest hardness values with more than 1600 HV1. By X-ray diffraction, it was possible to ascribe the lower hardness values of TiC and TiN implants to high amounts of retained austenite in the metal matrix. By implanting TiB2, the formation of retained austenite was successfully suppressed due to the in-situ formation of TiC particles, which was proven by electron backscatter diffraction. In conclusion, all the implant materials are basically suitable for laser implantation on AISI D2 tool steel. However, TiB2 has shown the most promising results.
Gut kombiniert: Laser-Plasma-Auftragschweißen
Für den Schutz vor starkem abrasiven Verschleiß (Panzern) oder vor korrosiven Medien (Plattieren) sowie für das Auftragen einer Pufferlage können das Laser-Pulver-Auftragschweißen oder das Plasma-Pulver-Auftragschweißen zum Einsatz kommen. Während das Laser-Pulver-Auftragschweißen eine geringe thermische Belastung verspricht, ermöglicht das Plasma-Pulver-Auftragschweißen hohe Auftragraten. Wissenschaftler des Fraunhofer IPK erarbeiten neuartige Verfahrenskombinationen und Düsenkonzepte, die die jeweiligen Vorteile dieser bestehenden Verfahren vereinen und somit eine hohe Qualität und Effizienz garantieren. So kann beispielsweise die punktgenaue Energie der Laserstrahlung den Plasmalichtbogen stabilisieren und den Wärmeeintrag in das Bauteil besser kontrollieren.
Plasma-Transferred-Arc (PTA) welding is a process that enables high deposition rates, but also causes increased thermal 9 load on the component. Laser based Direct Energy Deposition (DED) welding, on the other hand, achieves a high level of 10 precision and thus comparatively low deposition rates, which can lead to high processing costs. Combining laser and arc 11 energy aims to exploit the respective advantages of both technologies. 12 In this study, different possibilities of this process combination are presented using a PTA system and a 2 kW disk laser. 13 This includes the combination in a common process zone as a highspeed plasma laser cladding technology (HPLC), which 14 achieves process speeds of 10 m/min. Besides that it is being examined whether a pre-running plasma arc can be used to 15 coat difficult-to-weld rail steel with a carbon content of 0.8 % due to a preheating effect. Furthermore, a smoothing of the 16 coating by a plasma arc following the laser is investigated.
The geometrical characteristics of the weld end crater are commonly used for the validation of numerical results in welding simulation. A semi-analytical model calculating the cooling stage of the welding process after the moving energy source is turned off has been developed. A solution for various combinations of heat sources and workpieces has been found. The theoretical limits for the heat transfer of the absorbed energy during cooling in a thin plate and a semi-infinite body were studied. It is shown that after turning off the energy source, an additional melting of the base material in longitudinal direction may occur. The developed technique is applied to complete-penetration keyhole laser beam welding of a 2 mm thick austenitic chromium-nickel 316L steel plate at a welding speed of 20 mm/s and a laser power of 2.3 kW. The results show a theoretical increase of the weld end crater length in comparison to the length of the steady-state weld pool of up to 19 %. A shift of the centre of the end crater, in which the solidification of the liquid metal ends, towards the tail of the end crater relative to the axis of the heat source at the time of its termination, was computed. The speed and the direction of crystallization of the molten material in the weld pool and the end crater were found to be different. A good agreement between the computational results and the welding experiments was achieved.
Die klassische Beurteilung der Heißrissneigung erfolgt bislang primär über das Prüfen der Schweißeignung des Werkstoffes. Die konstruktiven Bedingungen und bauteilspezifischen Beanspruchungszustände bleiben dabei meist unberücksichtigt. Die Vielzahl an Heißrisstests liefert dabei lediglich werkstoff-spezifische, empiri-sche Heißrissparameter, wie z. B. Gesamtrisslänge oder Gesamtrissanzahl. Eine Bauteilübertragbarkeit ist somit oftmals nicht gegeben, gerade wenn es sich um hochbeanspruchte Konstruktionsmerkmale des Leichtbaus handelt. Folglich sind mechanische Kenngrößen (risskritische Verformungen/Spannungen) zu identifizieren, die sicher und unter realitätsnahen Bedingungen erfasst werden können. Diese dienen als Transfergrößen, um die Heißrissanfälligkeit geschweißter Bauteile mit typischen Konstruktionsmerkmalen zu bewerten.
Rapid localized heating and cooling during additive manufacturing using laser deposition method (LMD) lead to loss of dimensional accuracy as well as cracking of built parts. Finite-Element welding simulations allow prediction of geometrical deviations and accumulated residual stresses as well as their optimization before conducting experiments. Due to the great length of stacked welds, calculation times for fully transient thermomechanical simulations are currently long, the calculation stability suffers from the high number of contact bodies in the model and the modelling effort is high, as the geometries need to be sliced and positioned layer-wise.
In this contribution, an integrated modelling approach is demonstrated for a thin-walled LMD component made from 30 layers of 1.4404 (316L) stainless steel: Instead of the layer-by-layer modelling strategy commonly found in the literature, the whole component mesh is kept in one piece and the fully transient, layer-by-layer material deposition is implemented via element sets. In contrast to prior simulations, nonlinear contact between the layers does not have to be considered, significantly decreasing calculation times. The calculated distortions are compared to recently published, in-situ digital image correlation (DIC) measurements as well as numerical simulations conducted with the established layer-wise modelling strategy to judge result quality. Finally, the improvement in calculation time and ease-of-use is compared between both modelling approaches and conclusions regarding future usage for industrial-scale components are drawn.
Die wasserstoffunterstützte Schädigung von Komponenten und Bauteilen aus Stahl ist ein Phänomen, welches seit vielen Jahrzehnten bekannt ist und untersucht wird. Eine Vielzahl von Quellen (z.B. Schutzgas oder Feuchtigkeit beim Schweißen, Reinigung von Metallen in Säurebädern, galvanischer oder kathodischer Schutz) ermöglicht die Wasserstoffaufnahme in den Stahl. Der im Gitter gelöste oder an Wasserstoffhaftstellen (Versetzungen, Grenzflächen, Poren, etc.) getrappte Wasserstoff diffundiert aufgrund von Konzentrationsgradienten oder getrieben durch Spannungs- bzw. Dehnungsgradienten durch das vorliegende Gefüge, wo er in Kombination mit einwirkenden Beanspruchungen (äußere Last oder Eigenspannungen) eine lokale, signifikante Degradation der mechanisch-technologischen Eigenschaften bewirken kann.
Die dazu entwickelten und allgemein anerkannten Schädigungsmodelle gehen unter anderem von einem Einfluss des Wasserstoffs auf die Versetzungsentstehung und Versetzungsbeweglichkeit aus. Des Weiteren wird angenommen, dass Wasserstoff nicht nur durch Diffusion im Gitter transportiert wird, sondern auch an Versetzungen angehaftet ist und sich mit diesen im Falle plastischer Verformung mitbewegt.
Wasserstoff hat im Vergleich zu den meisten üblichen Legierungselementen von Stahl (Eisen, Chrom, Kohlenstoff) einen großen Wechselwirkungsquerschnitt für kalte und thermische Neutronen, was Neutronenradiographie und -tomographie zu geeigneten bildgebenden Verfahren zur lokalen Detektion von Wasserstoffakkumulationen in Stahl macht.
Elektrochemisch mit Wasserstoff beladene Zugproben aus supermartensitischem Stahl wurden vor und nach dem Zugversuch an der ANTARES beamline am FRM II polychromatisch radiographiert bzw. tomographiert. Die Bruchoberfläche der Probe wurde zusätzlich rasterelektronenmikroskopisch charakterisiert. Die Fraktographien der Bruchoberfläche in Verbindung mit den durch die Tomographie gewonnenen Informationen zu Wasserstoffansammlungen zeigen, dass nach dem Bruch auch in duktilen Versagensbereichen untypischerweise vermehrt Wasserstoff zu finden ist.
A novel approach for the reconstruction of an equivalent volumetric heat source from a known weld pool shape is proposed. It is based on previously obtained weld pool geometries from a steady-state thermo-fluid dynamics simulation. Hereby the weld pool dimensions are obtained under consideration of the most crucial physical phenomena, such as phase transformations, thermo-capillary convection, natural convection and temperature-dependent material properties. The algorithm provides a time and calibration efficient way for the reproduction of the weld pool shape by local Lamé curves. By adjusting their parameters, the identification of the finite elements located within the weld pool is enabled. The heat input due to the equivalent heat source is assured by replacing the detected nodes’ temperature by the melting temperature.
The model offers variable parameters making it flexible and adaptable for a wide range of workpiece thicknesses and materials and allows for the investigation of transient thermal effects, e.g. the cooling stage of the workpiece. The calculation times remain acceptably short especially when compared to a fully coupled process simulation. The computational results are in good agreement with performed complete-penetration laser beam welding experiments.
A novel approach for the reconstruction of an equivalent volumetric heat source from a known weld pool shape is proposed. It is based on previously obtained weld pool geometries from a steady-state thermo-fluid dynamics simulation. Hereby the weld pool dimensions are obtained under consideration of the most crucial physical phenomena, such as phase transformations, thermo-capillary convection, natural convection and temperature-dependent material properties. The algorithm provides a time and calibration efficient way for the reproduction of the weld pool shape by local Lamé curves. By adjusting their parameters, the identification of the finite elements located within the weld pool is enabled. The heat input due to the equivalent heat source is assured by replacing the detected nodes’ temperature by the melting temperature.
The model offers variable parameters making it flexible and adaptable for a wide range of workpiece thicknesses and materials and allows for the investigation of transient thermal effects, e.g. the cooling stage of the workpiece. The calculation times remain acceptably short especially when compared to a fully coupled process simulation. The computational results are in good agreement with performed complete-penetration laser beam welding experiments.
The weld pool dynamics and shape play a fundamental role in keyhole mode welding. The presented work aims the experimental and numerical investigation of the influence of the weld pool characteristics on the formation of hot cracking. The experimental procedure allows recording the molten pool in the longitudinal section of a butt joint configuration of 15 mm thick structural steel and transparent quartz glass by using a high-speed video camera and two thermal imaging cameras. The observations show that the dimensions of the weld pool vary depending on the depth. The regions close to the surface form a teardrop-shaped weld pool. A bulge-region and its temporal evolution are observed approximately in the middle of the depth of the weld pool, where hot cracking appears. A numerical framework including models for the weld pool dynamics, global temperature field, transient stress state, crystal growth, diffusion and macro-segregation and subroutines for their one-way couplings is developed. The numerically obtained and experimentally observed results are in a good agreement. It is shown that the bulge-region leads to a delay in the solidification behavior, increased temporal tensile stresses and accumulation of impurities in the defect region and hence enhance the probability of hot cracking formation.
The weld pool dynamics and shape play a fundamental role in keyhole mode welding. The presented work aims the experimental and numerical investigation of the influence of the weld pool characteristics on the formation of hot cracking. The experimental procedure allows recording the molten pool in the longitudinal section of a butt joint configuration of 15 mm thick structural steel and transparent quartz glass by using a high-speed video camera and two thermal imaging cameras. The observations show that the dimensions of the weld pool vary depending on the depth. The regions close to the surface form a teardrop-shaped weld pool. A bulge-region and its temporal evolution are observed approximately in the middle of the depth of the weld pool, where hot cracking appears. A numerical framework including models for the weld pool dynamics, global temperature field, transient stress state, crystal growth, diffusion and macro-segregation and subroutines for their one-way couplings is developed. The numerically obtained and experimentally observed results are in a good agreement. It is shown that the bulge-region leads to a delay in the solidification behavior, increased temporal tensile stresses and accumulation of impurities in the defect region and hence enhance the probability of hot cracking formation.
The geometrical characteristics of the weld end crater are commonly used for the validation of numerical results in welding simulation. A semi-analytical model calculating the cooling stage of the welding process after the moving energy source is turned off has been developed. A solution for various combinations of heat sources and workpieces has been found. The theoretical limits for the heat transfer of the absorbed energy during cooling in a thin plate and a semi-infinite body were studied. It is shown that after turning off the energy source, an additional melting of the base material in longitudinal direction may occur. The developed technique is applied to complete-penetration keyhole laser beam welding of a 2 mm thick austenitic chromium-nickel 316L steel plate at a welding speed of 20 mm/s and a laser power of 2.3 kW. The results show a theoretical increase of the weld end crater length in comparison to the length of the steady-state weld pool of up to 19 %. A shift of the centre of the end crater, in which the solidification of the liquid metal ends, towards the tail of the end crater relative to the axis of the heat source at the time of its termination, was computed. The speed and the direction of crystallization of the molten material in the weld pool and the end crater were found to be different. A good agreement between the computational results and the welding experiments was achieved.
The present study deals with the experimental characterization of short crack propagation in SLM (selective-laser-melting) manufactured stainless steel. More specifically, the determination of cyclic R-curves is discussed. This describes the dependency of the crack propagation threshold on crack growth during the short crack propagation stage. For metals, the threshold, starting at a material-intrinsic value, increases until it reaches a value independent of the crack length due to crack closure phenomena which build up at that stage. The cyclic R-curve, when used in the frame of a cyclic R curve analysis, characterizes the resistance of a material to fatigue crack growth and the ability to arrest a physically short crack. Thus, it is the link between classical fatigue and fracture mechanics. In the high-cycle-fatigue range, the short crack propagation stage dominates the overall lifetime, i.e., the number of cycles until failure. Below the fatigue limit crack arrest of hitherto propagable micro-cracks will occur. The effort for the experimental characterization of the short fatigue crack propagation behavior and the cyclic R-curve is very high compared to experiments on long crack propagation. A very exact measurement of crack extension is required, since small increments need to be depicted. Pre-cracking must leave a closure free initial crack, since closure must be build up only by the cyclic R-curve. The closure-free status is achieved by compression pre-cracking. The aim of the present study is an insight into the influence of an AM process on the short crack propagation threshold. Cyclic R-curves are experimentally determined at different load-ratios for 316L austenitic steel specimens produced by SLM and conventional manufacturing. Residual stresses are measured in the crack plane and their influence on the cyclic R-curve is discussed.
Residual stresses count among the most limiting factors in the application of additively manufactured materials in safety relevant components subject to cyclic loading. The source of such stresses is inherent in the manufacturing Laser Beam Melted (LBM) process due to rapid cooling and solidification and their distribution in the specimen or component is not homogeneous, but it is usually characterized by high gradients. Moreover, the magnitude of the residual stress field depends very much on the orientation, being higher in the build direction.
Many works in the literature advise to carry out a post-LBM treatment to relieve residual stresses, even though particular attention should be paid in choosing the parameters for the thermal treatment in order to avoid microstructural/phase transformations, which would affect greatly the mechanical properties of the material.
Particularly in the case of the austenitic stainless steel 316L, it has been shown that annealing at high temperatures (above 900°C), besides relieving the residual stresses, may cause recrystallization, grain growth and even phase transformation. In contrast, if a too low annealing temperature is chosen in order to preserve the microstructure, the residual stresses cannot be completely relieved.
This work aims to address the effect of residual stresses on short and long crack propagation for SEN(B) specimens made of 316L fabricated by LBM, in which notches have been machined by electro-discharge machining (EDM) on the mid-plane, perpendicular to the build direction. The specimens underwent different annealing treatments in inert atmosphere, in which the maximum temperature has been varied up to 900°C. The amount of residual stresses and their distribution has been measured by X-ray and neutron diffraction and the specimens have been subject to cyclic loading in a resonant testing machine. The tests show a massive influence of the residual stresses in the build direction on the resistance to fatigue crack propagation of additively manufactured 316L. Finally, a comparison with conventionally manufactured 316L is presented.
The present study deals with the experimental characterization of short crack propagation in SLM (selective-laser-melting) manufactured stainless steel. More specifically, the determination of cyclic R-curves is discussed. This describes the dependency of the crack propagation threshold on crack growth during the short crack propagation stage. For metals, the threshold, starting at a material-intrinsic value, increases until it reaches a value independent of the crack length due to crack closure phenomena which build up at that stage. The cyclic R-curve, when used in the frame of a cyclic R curve analysis, characterizes the resistance of a material to fatigue crack growth and the ability to arrest a physically short crack. Thus, it is the link between classical fatigue and fracture mechanics. In the high-cycle-fatigue range, the short crack propagation stage dominates the overall lifetime, i.e., the number of cycles until failure. Below the fatigue limit crack arrest of hitherto propagable micro-cracks will occur. The effort for the experimental characterization of the short fatigue crack propagation behavior and the cyclic R-curve is very high compared to experiments on long crack propagation. A very exact measurement of crack extension is required, since small increments need to be depicted. Pre-cracking must leave a closure free initial crack, since closure must be build up only by the cyclic R-curve. The closure-free status is achieved by compression pre-cracking. The aim of the present study is an insight into the influence of an AM process on the short crack propagation threshold. Cyclic R-curves are experimentally determined at different load-ratios for 316L austenitic steel specimens produced by SLM and conventional manufacturing. Residual stresses are measured in the crack plane and their influence on the cyclic R-curve is discussed.
The assessment of high cycle fatigue in additive manufactured (AM) components is a challenge due to complex microstructure, anisotropic material behavior, residual stresses and porosity / lack-of-fusion defects. Due to the statistical distribution of defects, a high scatter band of S-N-curves is expected. The fracture mechanics-based fatigue assessment of additive manufactured components must consider the propagation of short cracks emanating from defects.
In this work, the fatigue crack propagation resistance in the short and large crack regimes of additive and conventionally manufactured AISI 316L stainless steel is examined experimentally based on the cyclic R-curve. However, remaining residual stresses in the AM specimen lead to unexpected and dramatic crack-growth during the pre-cracking procedure.
Die Entstehung von Erstarrungsrissen stellt ein in der Schweißtechnik lange bekanntes Problem dar. Diese prozessseitige Herausforderung ist selbst nach dem heutigen Wissenstand schwer zu beherrschen und gefährdet die Sicherheit der gefügten Bauteile. Dieses Problem erweist sich als besonders schwierig beim einlagigen Hochleistungslaserstrahlschweißen von Metallteilen hoher Blechstärke, welche eine essentielle Rolle in vielerlei industriellen Anwendungen spielen.
Gegenstand des hier bearbeiteten Forschungsprojekts war die numerische Untersuchung der Entstehungsmechanismen des Mittelrippendefekts beim Laserstrahlschweißen niedriglegierter Stähle. Mittels mehrerer gekoppelter Simulationen, für welche sowohl kommerziellen Softwares, wie COMSOL Multiphysics und ANSYS, als auch in-house entwickelte Programme eingesetzt worden sind, wurden die am stärksten ausgeprägten Mechanismen der Erstarrungsrissbildung und deren Zusammenspiel aufgeklärt. Begleitend zur Simulation wurden mehrere experimentelle Untersuchungen für den niedriglegierten Baustahl S355 an den Blechdicken 12 mm und 15 mm zu deren Validierung durchgeführt. Die Ergebnisse der Simulation des Kristallwachstums, der Diffusion und der Makroseigerung von Schwefel zeigen, dass die Risse für bestimmte Werkstoffe mit einer hohen Konzentration an Verunreinigungen
hauptsächlich metallurgisch bedingt entstehen können. Allerdings zeigte die Untersuchung des Einflusses der Ausbauchung (Bulging) im Mittenbereich der Schweißnaht auf das transiente Temperaturfeld und die transienten Transversalspannungen im risskritischen Bereich auch, dass die Rissbildung nicht nur metallurgisch bedingt und insbesondere auch für aus metallurgischer Sicht eigentlich rissunkritische Materialien auftreten kann.
Der Bulging-Bereich hat sich in der Untersuchung als besonders kritisch in allen Teilen der Simulationskette gezeigt. Die Einflüsse dieser geometrischen Besonderheit konnten bei allen physikalischen Teilaspekten der Problematik als heißrissinduzierend identifiziert werden. Von besonderer Bedeutung wird an dieser Stelle das erkannte Zusammenspiel dieser Einflüsse hervorgehoben. Bisher wurde die Bildung der Erstarrungsrisse hauptsächlich durch einen einzelnen physikalischen Aspekt, wie z.B. rein metallurgisch oder rein thermomechanisch bedingt, isoliert betrachtet. Die gewonnen Erkenntnisse über die Einflüsse des Bulgings auf die unterschiedlichen Teilaspekte zeigen deutlich, dass alle Entstehungsmechanismen auch die Schmelzbadgeometrie als Ursprung haben. Von dieser hängen die transienten Temperatur- und Spannungsfelder, die Kristallwachsrichtung, die Erstarrungsgeschwindigkeit und die Konzentration von Verunreinigungen, welche zur Entstehung von niedrigschmelzenden Phasen führt, ab.
Die Ergebnisse des Forschungsvorhabens sind vielfach in referierten Zeitschriften und auf Fachtagungen veröffentlicht worden. Hervorzuheben dabei ist der „Best Presentation Award“ auf der LANE Conference 2018 in Fürth, wodurch die Thematik einem breiten Publikum bekannt gemacht werden konnte.
Um den wachsenden Anforderungen an den stofflichen und konstruktiven Leichtbau sowie den Forderungen nach Ressourceneffizienz Rechnung zu tragen, werden in vielen Industriebranchen zunehmend höherfeste Feinkornbaustähle mit Streckgrenzen über 690 MPa eingesetzt. Allerdings werden mit zunehmender Festigkeit deutlich höhere Anforderungen an deren schweißtechnische Verarbeitung gestellt, da die Sensibilität gegenüber einer Herabsetzung der mechanischen Eigenschaften durch den beim Schweißen aufgenommenen diffusiblen Wasserstoff mit steigender Festigkeit zunehmen kann. In den vergleichenden Untersuchungen von konventionellem Übergangslichtbogen und modifiziertem Sprühlichtbogen bei reduziertem Nahtöffnungswinkel konnte gezeigt werden, dass die Schweißprozessparameter die in das Schweißgut eingebrachte Wasserstoffkonzentration beeinflussen. Grundsätzlich ist den erarbeiteten Ergebnissen zu entnehmen, dass im Schweißgut von Stumpfstoßverbindungen mit reduziertem Nahtöffnungswinkel erhöhte mittlere Wasserstoffkonzentrationen vorliegen. Diese können mit geeigneten Wärmeführungen signifikant reduziert werden. Dabei erwies sich eine Nachwärmprozedur aus der Schweißwärme heraus als zielführend. Zudem konnte der Effekt der Nachwärmprozedur anhand fremdbeanspruchter Kaltrissprüfungen nachgewiesen werden.
Der Vortrag gibt einen Überblick über Arbeiten im Rahmen der Themenstellung "photonische Prozessketten" des SimCoLas Projekts. Es werden Ergebnisse zur Datendurchgängigkeit der Prozesskette Umformen/Laserstrahlschweißen sowie erste Ergebnisse zu numerischen Schweißstruktursimulationen zur Bewertung der Heissrisskritikalität eines tiefgezogenen Napfes beim Laserstrahlschweißen gegeben.
Die Präsentation gibt einen Überblick bezüglich der von der BAM im Rahmen des SimCoLas Verbundprojekts durchgeführten Arbeiten zum Thema "Photonische Prozessketten". Die numerisch abgebildete Prozesskette Tiefziehen/Laserstrahlschweißen wird darsgestellt und diskutiert. Fokus der Diskussion ist die entwickelte Methodik zur konsistenten Abbildung numerischer Prozessketten mit der Applikation Heissrisskritikalität beim Laserstrahlschweißen.
The deep penetration laser beam welding (LBW) has developed to one of the most promising metal joining methods in the modern manufacturing industry. It has well-known advantages of good penetration capacity, low heat input, high reachable welding speed and low welding distortion in comparison to conventional arc welding techniques. However, there are still challenges in LBW making the realization of the advantages difficult, such as porosity or inhomogeneous element distribution when using filler material. The magnetohydrodynamics technique is a promising way to solve these issues by introducing a suitable electromagnetic field, and correspondingly Lorentz force, to control the Transport phenomena in the weld pool.
The underlying physics in wire feed laser beam welding with electromagnetic stirring were investigated numerically and experimentally. A three-dimensional transient heat transfer and fluid flow model coupled with dynamic keyhole, magnetic induction and element transport was developed for the first time. The electromagnetic behaviour as well as the temperature and velocity profiles, solidification parameters, keyhole evolution and element transport are calculated.
The model is well tested against the experimental results. The beneficial effects from electromagnetic stirring (element homogenization and grain refinement) are explained quantitatively using the numerical data and the results from high-speed imaging, OM, EDX and EBSD.
Laser beam welding of aluminum die casting is challenging. A large quantity of gases (in particular hydrogen) is absorbed by aluminum during the die-cast manufacturing process and is contained in the base material in solved or bound form. After re-melting by the laser, the gases are released and are present in the melt as pores. Many of these metallurgic pores remain in the weld seam as a result of the high solidification velocities. The natural (Archimedean) buoyancy is not sufficient to remove the pores from the weld pool leading to process instabilities and poor mechanical properties of the weld. Therefore, an electromagnetic (EM) system is used to apply an additional buoyancy component to the pores. The physical mechanism is based on the generation of Lorentz forces, whereby an electromagnetic pressure is introduced into the weld pool. The EM system exploits the difference in electrical conductivity between poorly conducting pores (inclusions) and the comparatively better conducting aluminum melt to increase the resulting buoyancy velocity of the pores. Within the present study, the electromagnetic supported degassing is investigated in dependence on the laser beam power, welding velocity and electromagnetic flux density. By means of a design of experiments a systematic variation of these parameters is carried out for partial penetration laser beam welding of 6 mm thick sheets of wrought aluminum alloy AlMg3 and die-cast aluminum alloy AlSi12(Fe) where the wrought alloy serves as a reference. The proportion of pores in the weld seams is determined using X-ray images, computed tomography (CT-) images and cross-section images. The results prove a significant reduction of the porosity up to 70 % for both materials as a function of the magnetic flux density
When assessing the performance of welded components residual stresses are vital. The possibilities of transferring the real boundary conditions of welding, which influence the residual stress, into the laboratory are highlighted in this contribution. The potentials of a test system specially developed for this purpose are demonstrated. The component design induces global process-, geometry- and material-dependent stresses, which can be simulated and quantified in the system. In addition, the resulting local residual stress distribution can be exactly determined with high spatial resolution with the aid of X-ray diffraction. Examples are presented of how the conditions to be found during production are simulated in the laboratory.
The potential of lowered surface features as well as the application of wear resistant coatings have been known for many years to improve the tribological behavior of forming tools. More recent studies also discuss the capability of protruded microfeatures for adjusting the tribological behavior between contacting surfaces. The demand for a high wear resistance of such structures as well as their economical and reliable production, however, often limits the industrial application. The laser implantation process can overcome these limitations. In contrast to conventional cw-laser dispersing processes, where the formation of uniform metal matrix composite layers is intended, this surface engineering technique aims to improve the tribological behavior of contacting surfaces by a localized dispersing of pre-placed hard ceramic particles. This enables the formation of deterministic textures composed of separated wear resistant dome- or ring-shaped microstructures (implants).
Since TaC shows very promising material properties for improving the wear resistance of tools exposed to severe operating conditions, this paper analyzes its suitability for pulsed laser implantation on X38CrMoV5-3 tool steel for the first time. In the experiments, the influence of the particles and the laser parameters (pulse power, pulse duration and focal diameter) on the material properties of the localized dispersed zones was studied by optical microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. The composite´s (micro-) hardness was measured and calculated by using a rule of mixture. Additionally, the influence of the laser parameters and the TaC particles on the geometrical properties of the implants was studied by optical microscopy and white light interferometry.
The results showed that defect-free implants with hardness values of ~900 HV1 can be obtained at the focal spot, since a localized dispersing of the TaC particles is possible using a pulsed millisecond laser. However, in dependence of the laser intensity, also a partial dissolution of the initial particles occurs. This leads to the precipitation of new dendritic TaC nanoparticles and to varying contents of retained austenite in the matrix. Both effects have a strong influence on the implant hardness and must be considert by the rule of mixture. Regarding the geometrical response it was pointed out that protruded microfeatures with heights up to 10 µm can be created. In comparison to laser remelted zones, the implanted zones showed significantly altered weld pool profiles due to the influence of the particles on the melt convection. A transition of the implant shape from predominantly dome-shaped to predominantly ring-shaped was observed for intensities >1.7∙106 W/cm2 due to the onset of the keyhole effect.
In additive manufacturing (AM) Laser Metal Deposition (LMD), parts are built by welding layers of powder feedstock onto a substrate. Applications for steel powders include forging tools and structural components for various industries. For large parts, the choice of tool-paths influences the build-rate, the part performance and the distortions in a highly geometry-dependent manner. With weld-path lengths in the range of hundreds of meters, a reliable, automated tool path generation is essential for the usability of LMD processes.
In this contribution, automated tool-path generation approaches are shown and their results are discussed for arbitrary geometries. The investigated path strategies are the classical approaches: “Zig-zag-” and “contour-parallel-strategies”. After generation, the tool-paths are automatically formatted into g-code for experimental build-up and ASCII for a numerical simulation model. Finally, the tool paths are discussed in regards to volume-fill, microstructure and porosity for the experimental samples.
This work presents a part of the IGF project 18737N “Welding distortion simulation” (FOSTA P1140)
Das DED Auftragschweißen ist ein additives Fertigungsverfahren für Metalle, bei dem das Material schichtweise auf ein Substrat aufgetragen wird. Die schnellen Temperaturzyklen rufen Spannungsgradienten im Bauteil hervor. Der schichtweise Aufbau der Bauteile verursacht eine anisotrope Mikrostruktur. Mittels nachgelagerter Wärmebehandlung können diese Effekte verringert werden. Im anschließenden Schritt der Prozesskette wird das additiv hergestellte Bauteil mittels Drahterodieren von dem Substrat abgetrennt.
In diesem Beitrag wird eine thermo-mechanische Simulation der gesamten Prozesskette vorgestellt, welche den additiven Aufbau, Wärmebehandlung und das Abtrennen vom Substrat beinhaltet. Anstelle der in der Literatur üblichen schichtweisen Modellierungsstrategie für die DED Simulation wird das gesamte Bauteil in einem Stück vernetzt und der vollständig transiente, schichtweise Materialauftrag über Elementgruppen realisiert. Im Gegensatz zu früheren Simulationen muss der nichtlineare Kontakt zwischen den Schichten nicht berücksichtigt werden, was die Rechenzeiten deutlich verkürzt.
Das Modell wurde validiert mittels Abgleiches des Verzugs aus Simulation und Experiment.
Die Proben, bestehend aus DIN 1.4404 (AISI 316L), wurden nach jedem Prozessschritt 3D gescannt um den Verzug zu quantifizieren. Zusätzlich wurden Querschnitte und Härtetests nach Vickers von unterschiedlich behandelten Proben durchgeführt, um den Effekt der Wärmebehandlung auf die Mikrostruktur und die Härte des Bauteils zu untersuchen.
Приведено аналитическое решение задачи теплопроводности после прекращения действия подвижного источника теплоты для различных комбинаций источника и свариваемого тела. Показано, что после выключения источника возможно дополнительное плавление основного металла за счет перегретого жидкого металла сварочной ванны. Например, при лазерной сварке со сквозным проплавлением стальной пластины толщиной 2 мм длина кратера шва может быт на 19% больше установившейся длины сварочной ванны. Установлено, что центр кратера, в котором заканчивается затвердевание жидкого металла, смещен в сторону хвостовой части кратера относительно оси теплового источника в момент прекращения его действия. Скорость и направление кристаллизации металла сварочной ванны и кратера различны.
The introduction of an external magnetic field in laser beam welding can bring a lot of beneficial effects e. g. a homogeneous element distribution and grain refinement. However, the physical mechanism has not been fully revealed because it is difficult to describe the weld pool behavior quantitatively. In this work, the influence of an oscillating magnetic field in wire feed laser beam welding is studied using a three-dimensional transient multi-physical model. The magnetohydrodynamics, heat transfer, fluid flow, keyhole dynamics and element transport are all taken into consideration. Under suitable electromagnetic parameters, a significant electromagnetic stirring can be produced in the weld pool by the induced Lorentz force. It shows important influence on the keyhole stability and the fluid flow. The keyhole collapses more frequently at the upper part under the oscillating magnetic field. The forward and downward flow along the central plane of the weld pool is enhanced, which can bring the additional element from the filler wire to the root of the weld pool. There is a good agreement between the numerical results and the experiment results from the high-speed imaging, the optical micrograph and the energy dispersive X-ray spectroscopy. This work provides a fundamental understanding of the relationship between electromagnetic control, weld pool behavior and weld property.
Die Laserimplantation erlaubt die Herstellung verschleißbeständiger, erhabener Mikrostrukturen (Implants) auf Stahloberflächen durch ein diskontinuierliches Dispergieren von keramischen Partikeln mittels gepulster Laserstrahlung. Durch die flexible Anordnung separierter Implants zu komplexen Mustern erlaubt das Verfahren eine gezielte Oberflächenstrukturierung zur Beeinflussung des Reibungs- und Verschleißverhaltens. Insbesondere erwies sich Titandiborid (TiB2) als Implantationsmaterial für geeig-net, da eine Manipulation der Implantgeometrie in einem breiten Bereich vorgenommen werden konnte, ohne dass Materialdefekte wie Risse oder Poren auftraten.
Ziel der Untersuchungen war es, den Einfluss implantierter TiB2-Partikel auf die Materialeigenschaften von X153CrMoV12 zu ermitteln. Hierfür wurden im Rahmen der Arbeit die Laserparameter (Pulsleistung und -dauer) in einem breiten Parameterfeld variiert und vergleichende Untersuchungen an TiB2 implan-tierten Zonen sowie an punktuell umschmelzstrukturierten Zonen durchgeführt. Die Ergebnisse zeigen, dass eine reine Umschmelzstrukturierung zu einer deutlichen Reduktion der Oberflächenhärte aufgrund erhöhter Restaustenitgehalte (γR) führt. Im Gegensatz dazu führt das Laserimplantieren von TiB2-Partikeln zu einer deutlichen Härtesteigerung in den kuppel- oder ringförmigen Implants. Härtewerte von bis zu 1800 HV1 resultieren aus dispergierten TiB2-Primärpartikeln sowie in-situ ausgeschiedenen Se-kundärphasen, durch die der Restaustenitanteil zudem deutlich reduziert wird.
Solidification cracking of metals is a well-researched topic in the field of welding science. A material’s susceptibility to solidification cracking can be tested using numerous different specialized test procedures, one of which is the Modified Varestraint-/Transvarestraint test (MVT). It was developed at BAM in 1982 and is internationally standardised. Over the decades, this test has been extensively used to characterise the solidification cracking resistance of many different materials.
The present study was conducted to further investigate the influences of the standardised MVT testing parameters, as well as the characteristics of evaluation methods on the results. Several different high alloyed martensitic LTT (low transformation temperature) filler materials, CrNi and CrMn type, were used. In previous pilot studies, these alloys have shown a rather distinctive solidification cracking behaviour.
First, the effects of different process parameter sets on the solidification cracking response were measured using the standard approach. Subsequently, μCT scans were performed on the specimens. The results consistently show sub surface cracking, to significant, yet varying extents. Different Primary solidification types were found using WDX-analysis, an aspect that is regarded to be the main difference between the CrNi- and CrMn-type materials and their cracking characteristics.
Results show that when it comes to testing of modern high-performance alloys, one set of standard MVT testing parameters might not be equally suitable for all materials. Also, to properly accommodate different solidification types, sub-surface cracking has to be taken into account.
The two-part paper provides an overview on the state-of-the-art in the application of engineering fracture mechanics to weldments. This, of course, cannot be exhaustive but is limited to butt and fillet welds with crack initiation at weld toes. In the present first pari, the authors briefly focus on the susceptibility of welds to cracks and other defects. Following this, they discuss in more detail the consequences of material inhomogeneity across the weld for fracture mechanics. Inhomogeneity causes scatter in fracture toughness and strength mis-match effects which both have to be considered in fracture toughness testing, crack driving force determination and fracture assessment of welded components. Part 2 of the paper series will add a discussion of welding residual stresses and questions of applying fracture mechanics to residual as well as total lifetime estimation of welds under cyclic loading.
The shape of the parts, created by the technology of direct laser metal deposition (DLMD), is influenced by various parameters, for example, the power and diameter of the laser source spot. The contribution of energy from the laser affects the temperature distribution in the formed layers. The changing temperature in the working area entails a change in the geometry of the layers and affects the stability of the process. In this paper, experiments on the measurement of temperature cycles in the DLMD process with different directions of the filling track are carried out. An infrared camera was used to measure thermal cycles. The calibration of the acquired data (i.e. correspondence table between the intensity of thermal radiation of the material and the absolute temperature) was done with help of two-color pyrometer ex situ and in situ measurements. The experiments are carried out on two materials 316L and Inconel 718. The effect of the maximum temperature on the layer height is shown, and thermal cycles in the formation of layers for different filling strategies are presented.
The shape of the parts, created by the technology of direct laser metal deposition (DLMD), is influenced by various parameters, for example, the power and diameter of the laser source spot. The contribution of energy from the laser affects the temperature distribution in the formed layers. The changing temperature in the working area entails a Change in the geometry of the layers and affects the stability of the process. In this paper, experiments on the measurement of temperature cycles in the DLMD process with different directions of the filling track are carried out. An infrared camera was used to measure thermal cycles. The calibration of the acquired data (i.e. correspondence table between the intensity of thermal radiation of the material and the absolute temperature) was done with help of two-color pyrometer ex situ and in situ measurements. The experiments are carried out on two materials 316L and Inconel 718. The effect of the maximum temperature on the layer height is shown, and thermal cycles in the formation of layers for different filling strategies are presented.
Supermartensitic stainless steels (SMSS) are a commonly used material nowadays for building offshore structures, i.e. pipelines in the oil and gas industry. The harsh and corrosive environments in oil and gas applications require the correct combination of alloys to attain the desired properties of steel, including high strength and good corrosion properties, even in severe sour service conditions. Welding is the most commonly used method in joining offshore components, depending on requirements requiring strength or fitting. It has been shown that the heat affected zone (HAZ) is more susceptible to certain types of corrosion, including pitting corrosion, especially during severe sour service where a high pH and lower H2S values in the flow medium can lead to pitting corrosion in the HAZ of welded structures. Subsequent hydrogen uptake in the pits can cause cracks to initiate and propagate, leading to rupture of pipelines or catastrophic failures of structures, even at low mechanical loads. Offshore standards allow a certain amount of corrosion, including pitting, to be present before action is required, however the extent of pitting corrosion is not identified by performing visual inspection alone as the subsurface pit diameter may be vastly greater than the pit diameter at the surface. The critical conditions which lead to crack initiation and propagation from a pit with hydrogen uptake are currently not known. Therefore, pitting corrosion and subsequent crack initiation are a danger to the safety of structures. The interest in this phenomenon has resulted in many experimental studies and numerical simulations.
Several numerical models of pitting corrosion and hydrogen uptake resulting in crack initiation are already in existence, but these two phenomena are regularly modelled individually. Thus, a model enabling simulation of both phenomena simultaneously would be of great benefit. Hence, the goal of this study is to develop a model enabling simulation of pit growth and crack initiation, considering hydrogen uptake in the pit from a corrosive environment.
As a first step, this paper presents an investigation into various parameters, which influence crack initiation at pits. These crack critical parameters include: pit geometry, pit location, mechanical load and hydrogen transport into the microstructure. The results will help to identify critical conditions for crack initiation starting at the pit and developing measures to avoid hydrogen assisted cracking (HAC).
Plasma-Transferred-Arc (PTA) welding is a process that enables high deposition rates, but also causes increased thermal load on the component. Laser based Direct Energy Deposition (DED) welding, on the other hand, achieves a high level of precision and thus comparatively low deposition rates, which can lead to high processing costs. Combining laser and arc energy aims to exploit the respective advantages of both technologies.
In this study, different possibilities of this process combination are presented using a PTA system and a 2 kW disk laser. This includes the combination in a common process zone as a highspeed plasma laser cladding technology (HPLC), which achieves process speeds of 10 m/min. Besides that it is being examined whether a pre-running plasma arc can be used to coat difficult-to-weld rail steel with a carbon content of 0.8 % due to a preheating effect. Furthermore, a smoothing of the coating by a plasma arc following the laser is investigated.
The presentation provides a discussion and damage tolerant assessment of metallic AM components. In the focus are problems of the determination of representative material data, the effect of material defects and residual stresses. Starting with the actual state-of-the-art in the field, options and possibilities of a damage tolerant design for AM are discussed.
This work aims to find the thermal cycles during and after fusion welding through simulation by first calculating the resulting local temperature field in the quasi-stationary part of the process. Here complete-penetration keyhole laser beam welding with a laser power of 18 kW on a 15 mm thick slab of a low-alloyed steel at a welding speed of 2 m/min is considered. In order to physically depict the laser material interaction a multi-physics numerical model including the effects of phase transformation, thermo-capillary convection, natural convection and temperature-dependent material properties up to evaporation temperature is developed. It uses a fixed keyhole geometry with a right truncated circular cone shape to introduce the laser beam energy to the workpiece. In a subsequent study, the resulting local temperature field is then used as an equivalent heat source in order to predict the unsteady thermal cycle during and after fusion welding. The translational movement of the laser beam through the workpiece is represented by a moving mesh approach. For the simulation, stationary heat transfer and fluid dynamics are described by a system of strongly coupled partial differential equations. These are solved with the commercial finite element software COMSOL Multiphysics 5.0. The results of the numerical simulation are validated by experiments, where the weld bead shapes and the thermal cycles show good correlation.
Neutron cameras allow visualizing hydrogen distributions with radiographic or tomographic imaging methods in iron (and steel) and many other metals. The necessary contrast between hydrogen and these metals stems from the high difference in the total neutron cross section of both elements. This allows, e.g., the in situ measurement of hydrogen mass flow inside cm thick metal samples with a temporal resolution of at best 10 s using neutron radiography as well as the quantitative measurement of hydrogen accumulations, e.g., at the crack’s inner surfaces in hydrogen embrittled iron samples with neutron tomography. This new quality of the information on a micrometer scale allows new insights for the analysis of hydrogen-assisted damage mechanisms. Further, this method is non-destructive and provides local information in situ and in three dimensions with a spatial resolution of 20 μm - 30 μm. In this contribution, we give a short historical overview of neutron imaging and show examples that demonstrate the spatial and temporal resolution of the neutron radiography and tomography methods in order to visualize and quantify hydrogen accumulations, absorption processes, and diffusion. The examples are taken from the works of researchers dealing with titanium, palladium, zirconium, and iron or steel. More detailed descriptions of the experimental and analytic procedures are given for hydrogen detection using radiography and tomography on iron and steel samples.
In this study, the influence of the welding speed and the arc power on the solidification crack formation for partial penetration laser hybrid welded Thick-Walled plates were investigated. Experimentally, a linear correlation between the welding velocity and the crack number was observed. That is by reducing the welding velocity the crack number was reduced.
The reduced welding velocity showed a strong impact on stress, as the model demonstrated a very lower stress amount in comparison to the reference case. The reduction of the welding speed could be a helpful technique to reduce the hot cracking. The wire feed speed showed a very slight influence on the crack formation. That can be returned to the large distance between the critical region for cracking and the arc region.
In this study, the influence of the welding speed and the arc power on the solidification crack formation for partial penetration laser hybrid welded Thick-Walled plates were investigated. Experimentally, a linear correlation between the welding velocity and the crack number was observed. That is by reducing the welding velocity the crack number was reduced.
The reduced welding velocity showed a strong impact on stress, as the model demonstrated a very lower stress amount in comparison to the reference case. The reduction of the welding speed could be a helpful technique to reduce the hot cracking. The wire feed speed showed a very slight influence on the crack formation. That can be returned to the large distance between the critical region for cracking and the arc region.
In this study, the influence of the welding speed, the arc power and the laser focal position on the solidification crack formation for partial penetration laser hybrid welded thick-walled plates were investigated. Experimentally, a direct correlation between the welding speed and the crack number was observed. That is by reducing the welding velocity the crack number was decreased. The focal position shows also a significant influence on the crack number. Since by focusing the laser on the specimen surface, the crack number has been significantly diminished.
The wire feed speed showed a very slight influence on the crack formation. That is due to the large distance between the critical region for cracking and the arc region.
The numerical model shows a high-stress concentration in the weld root for both components (vertical and transversal).
Numerically, the reduced welding speed showed a strong impact on stress, as the model demonstrated a lower stress amount by decreasing the welding speed. The solidification cracking in the weld root is a result of interaction between metallurgical and geometrical and thermomechanical factors.
Laser-induced periodic surface structures (LIPSS, ripples) with ~500–700 nm period were produced on titanium alloy (Ti6Al4V) surfaces upon scan processing in air by a Ti:sapphire femtosecond laser. The tribological performance of the surfaces were qualified in linear reciprocating sliding tribological tests against balls made of different materials using different oilbased lubricants. Extending our previous work, we studied the admixture of the additive 2-ethylhexyl-zinc-dithiophosphate to a base oil containing only anti-oxidants and temperature stabilizers. The presence of this additive along with the variation of the chemical composition of the counterbodies allows us to explore the synergy of the additive with the laseroxidized nanostructures.
Höherfeste Feinkornbaustähle mit Streckgrenzen ≥ 690 MPa neigen bei der schweißtechnischen Fertigung zur Bildung von wasserstoffunterstützten Kaltrissen. Diese Mikrorisse entstehen aus der kritischen Interaktion von lokalem risskritischem Gefüge, lokal erhöhter Wasserstoffkonzentration und lokal erhöhter Beanspruchung bzw. Dehnung. Aktuell gültigen Regelwerken zur schweißtechnischen Verarbeitung höherfester Feinkornbaustähle sind Anweisungen zu entnehmen, um das Risiko einer Kaltrissbildung zu minimieren. Die Weiterentwicklung der Werkstoffe (Legierungskonzepte)und neuer Lichtbogenprozesse zur Verringerung der Nahtöffnungswinkel bei gleichzeitiger Erhöhung der Abschmelzleistung (modifizierter Sprühlichtbogen) brachten jedoch auch neue Fragen hinsichtlich der Wasserstoffmenge in der Schweißverbindung nach dem Schweißen und der Kaltrissempfindlichkeit mit sich. Mikrolegierungselemente (V, Nb und Ti) bedingen bspw. eine verlangsamte Wasserstoffdiffusion. Schweißverbindungen unter reduziertem Nahtöffnungswinkel beinhalten höhere Wasserstoffkonzentrationen. Dieser Beitrag zeigt den Einfluss von Einbrandprofil (höhere Abschmelzleistung) auf die Kaltrissempfindlichkeit an einem höherfesten Feinkornbaustahl mit artähnlichem mikrolegierten Schweißzusatzwerkstoff anhand des fremdbeanspruchten Implant-Tests.
Höherfeste Feinkornbaustähle mit Streckgrenzen ≥ 690 MPa neigen bei der schweißtechnischen Fertigung zur Bildung von wasserstoffunterstützten Kaltrissen. Diese Mikrorisse entstehen aus der kritischen Interaktion von lokalem risskritischem Gefüge, lokal erhöhter Wasserstoffkonzentration und lokal erhöhter Beanspruchung bzw. Dehnung. Aktuell gültigen Regelwerken zur schweißtechnischen Verarbeitung höherfester Feinkornbaustähle sind Anweisungen zu entnehmen, um das Risiko einer Kaltrissbildung zu minimieren. Die Weiterentwicklung der Werkstoffe (Legierungskonzepte) und neuer Lichtbogenprozesse zur Verringerung der Nahtöffnungswinkel bei gleichzeitiger Erhöhung der Abschmelzleistung (modifizierter Sprühlichtbogen) brachten jedoch auch neue Fragen hinsichtlich der Wasserstoffmenge in der Schweißverbindung nach dem Schweißen und der Kaltrissempfindlichkeit mit sich. Mikrolegierungselemente (V, Nb und Ti) bedingen bspw. eine verlangsamte Wasserstoffdiffusion. Schweißverbindungen unter reduziertem Nahtöffnungswinkel beinhalten höhere Wasserstoffkonzentrationen. Dieser Beitrag zeigt den Einfluss von Einbrandprofil (höhere Abschmelzleistung) auf die Kaltrissempfindlichkeit an einem höherfesten Feinkornbaustahl mit artähnlichem mikrolegierten Schweißzusatzwerkstoff anhand des fremdbeanspruchten Implant-Tests.
Hybrid laser-arc welding offers many advantages, such as deep penetration, good gap bridge-ability, and low distortion due to reduced heat input. The filler wire which is supplied to the process is used to influence the microstructure and mechanical properties of the weld seam.
A typical problem in deep penetration high-power laser beam welding with filler wire and hybrid laser-arc welding is an insufficient mixing of filler material in the weld pool, leading to a non-uniform element distribution in the seam. In this study, oscillating magnetic fields were used to form a non-conservative component of the Lorentz force in the weld pool to improve the element Distribution over the entire thickness of the material. Full penetration hybrid laser-arc welds were performed on 20-mm-thick S355J2 steel plates with a nickel-based wire for different arrangements of the oscillating magnetic field. The Energy-dispersive X-ray spectroscopy (EDS) data for the distribution of two tracing elements (Ni and Cr) were used to analyze the homogeneity of dilution of the filler wire.
With a 30° turn of the magnetic field to the welding direction, a radical improvement in the filler material distribution was demonstrated. This would lead to an improvement of the mechanical properties with the use of a suitable filler wire.
The advantage of selective laser melting (SLM) is its high accuracy and geometrical flexibility.
Because the maximum size of the components is limited by the process chamber, possibilities must be found to combine several parts manufactured by SLM. An application where this is necessary, is, for example, the components of gas turbines, such as burners or oil return pipes, and inserts, which can be joined by circumferential welds. However, only a few investigations to date have been carried out for the welding of components produced by SLM. The object of this paper is, therefore, to investigate the feasibility of laser beam welding for joining SLM tube connections made of nickel-based alloys.
For this purpose, SLM-manufactured Inconel 625 and Inconel 718 tubes were welded with a Yb:YAG disk laser and subsequently examined for residual stresses and defects. The results showed that the welds had no significant influence on the residual stresses. A good weld quality could be achieved in the seam circumference. However, pores and pore nests were found in the final overlap area, which meant that no continuous good welding quality could be accomplished. Pore formation was presumably caused by capillary instabilities when the laser power was ramped out.
In special cases of angle beam ultrasonic measurement – e.g. defect detection in hot solids as well as flow measurement of liquid gases or energy storage mediums – the applied transducer has to withstand extreme temperatures. Since the irradiation angle into the specific material is determined not only by wedge design, but also by the speed of sound in both the wedge material and the tested object, the developer must take into account the speed of the wave propagation in a wedge material over the whole temperature range of transducers application.
In this study we investigate the temperature dependence of the speed of longitudinal wave propagation in 10 different materials in the range from -200 °C to 400 °C. The investigated materials belong to different material classes (ceramics, glass, as well as ferrous and non-ferrous metals) and are all temperature-resistant up to at least 600 °C, and therefore applicable as wedge materials in an ultrasonic transducer for use at extreme temperatures.
In special cases of angle beam ultrasonic measurement the applied transducer has to withstand extreme temperatures. Since the irradiation angle depends on the speed of sound in both the wedge material and the tested object, the developer must take into account the speed of Sound in a wedge material over the whole temperature range of transducers application.
The Modified Varestraint Transvarestraint (MVT) test was used to investigate the solidification cracking susceptibility of an unstabilised, a Nb-stabilised and two (Ti + Nb) dual-stabilised ferritic stainless steels. Two different welding speeds of 6 and 3 mm/s using autogenous gas tungsten arc welding were employed. At the welding speed of 6 mm/s, the high-content (Ti + Nb) steel was resistant and the Nb-stabilised steel was marginally susceptible to solidification cracking. At the welding speed of 3 mm/s, the Nb and the high (Ti + Nb) steels were found to be marginally susceptible to solidification cracking while the unstabilised and low-content (Ti + Nb) grades were resistant to solidification cracking. The weld metal microstructures transverse to the welding direction revealed columnar grains in all the samples for both welding speeds. The ferritic stainless steels were generally resistant to solidification cracking, except the Nb-stabilised steel, which was marginally susceptible to solidification cracking.
The presented study deals with the performing and mechanical testing of single pass hybrid laser-arc welds (HLAW) on 25 mm thick plates made of steel grade S355J2. One of the challenges have to be solved at full penetration HLAW of thick plates is the drop formation occurring due to the disbalances of the forces acting in the keyhole and on the melt pool surface. Such irregularities mostly limit the use of high-power laser beam welding or HLAW of thick-walled constructions. To overcome this problem, an innovative concept of melt pool support based on generating Lorentz forces in the weld pool is used in this work. This method allows to perform high quality welds without sagging even for welding of 25 mm thick plates in flat position at a welding speed of 0.9 m min-1. For the obtain of full penetrated welds a laser beam power of 19 kW was needed. A high V-impact energy of up to 160 J could be achieved at the test temperature of 0 °C. Even at the most critical part in the weld root an impact energy of 60 J in average could be reached. The tensile strength of the weld reaches that of the base material. An introduce of the HLAW process with electromagnetic support of the melt pool in the industrial practice is an efficient alternative to the time- and cost-intensive arc-based multi-layer welding techniques which are established nowadays for joining of thick-walled constructions.
With global increases in clean energy demand, the natural gas is gaining in importance. Pipelines are the safest and most cost-effective way of transporting natural gas. Due to high transport volume and resulting high operation pressure, the demand for ultra-high strength steel grades such as X120 is very strong. As a result of the fact that these steels are produced by thermo-mechanical controlled processing, the welding process must be selected accordingly. Based on investigations, a high heat input such as by submerged arc welding process leads to softening in the weld metal and loss of strength whereas pure laser beam welding results in high cooling rates and deteriorate toughness of the weld metal. The objective of this research is to investigate the influence of heat input to mechanical properties of hybrid laser-arc welded pipeline steels of grade X120. Test specimens with a thickness of 20 mm could be welded without preheating in a single-pass with different welding velocities to observe the largest possible parameter window of the heat input. The achieved V-notch impact energy for hybrid laser-arc welded samples was 144±37 J at a testing temperature of -40 °C. With a tensile strength of 930±4 MPa the requirements of API 5L was achieved. To prevent gravity drop-outs at the slow welding speeds, an electromagnetic weld pool support system was used, which works contactless and is based on generating Lorentz forces. It was therefore possible to control the cooling rate in order to meet the requirements of the mechanical properties. By adapting the electromagnetic weld pool support to the laser and laser hybrid welding process, the application potential of these technologies for industrial implementation can be drastically increased.
Single-pass Hybrid Laser Arc Welding of Thick Materials Using Electromagnetic Weld Pool Support
(2019)
Hybrid laser-arc welding process allows single-pass welding of thick materials, provides good quality formation of joints with minimal thermal deformations and a high productivity in comparison with arc-based welding processes. Nevertheless, thick-walled steels with a thickness of 20 mm or more are still multi-pass welded using arc welding processes, due to increased process instability by increasing laser power. One limitation factor is the inadmissible formation of gravity drop-outs at the root. To prevent this, an innovative concept of electromagnetic weld pool support is used in this study. With help of such system a stable welding process can be established for 25 mm thick steel plates and beyond. Sound welds could be obtained which are tolerant to gaps and misalignment of the welded parts. The adaptation of this system to laser and hybrid laser-arc welding process can dramatically increase the potential field of application of these technologies for real industrial implementation.
Results of experimental investigations of the relationship between laser-hybrid welding process parameters, type of the filler metal and the mechanical properties of the welds made from 9% nickel cryogenic steel X8Ni9 are discussed. The results contribute to the development and conversion in the industrial practice a new laser beam-based welding technology for the automated manufacturing of LNG tanks. The remarkable heterogeneity in the chemical composition of the weld metal as well as an insufficient impact toughness could be indicated by using austenitic filler wire. The most promising results were achieved by applying 11%Ni filler wire, which is similar to the base material. A correlation between impact toughness and wire feeding speed could be shown. The highest impact toughness was 134 J at -196°C. The laser-hybrid welds passed the tensile test. The failure stress of 720 MPa with a fracture location in the base metal was achieved for all samples tested.
Single-pass Hybrid Laser Arc Welding of Thick Materials Using Electromagnetic Weld Pool Support
(2019)
Hybrid laser-arc welding process allows single-pass welding of thick materials, provides good quality formation of joints with minimal thermal deformations and a high productivity in comparison with arc-based welding processes. Nevertheless, thick-walled steels with a thickness of 20 mm or more are still multi-pass welded using arc welding processes, due to increased process instability by increasing laser power. One limitation factor is the inadmissible formation of gravity drop-outs at the root. To prevent this, an innovative concept of electromagnetic weld pool support is used in this study. With help of such system a stable welding process can be established for 25 mm thick steel plates and beyond. Sound welds could be obtained which are tolerant to gaps and misalignment of the welded parts. The adaptation of this system to laser and hybrid laser-arc welding process can dramatically increase the potential field of application of these technologies for real industrial implementation.
The present work deals with the development of a strategy for the prevention of end crater defects in high-power laser welding of thick-walled circumferential welds. A series of experiments were performed to understand the influence of the welding parameters on the formation of end crater defects such as pores, cracks, excessive root-side drop-through and shrinkage cavities in the overlap area. An abrupt switch-off of the laser power while closing the circumferential weld leads to a formation of a hole which passes through the whole welded material thickness. A laser power ramp causes solidification cracks which are initiated on the transition from full-penetration mode to partial penetration. Strategies with a reduction of the welding speed shows a creation of inadmissible root sagging. Defocusing the laser beam led to promising results in terms of avoiding end crater defects. Cracks and pores in the overlap area could be effectively avoided by using defocusing techniques. A strategy for avoiding of end crater imperfections was tested on flat specimens of steel grade S355J2 with a wall thickness of 10 mm and then transferred on the 9.5 mm thick pipe sections made of high-strength steel X100Q.
The presented study deals with the performing and mechanical testing of single pass hybrid laser-arc welds (HLAW) on 25 mm thick plates made of steel grade S355J2. One of the challenges have to be solved at full penetration HLAW of thick plates is the drop formation occurring due to the disbalances of the forces acting in the keyhole and on the melt pool surface. Such irregularities mostly limit the use of high-power laser beam welding or HLAW of thick-walled constructions. To overcome this problem, an innovative concept of melt pool support based on generating Lorentz forces in the weld pool is used in this work. This method allows to perform high quality welds without sagging even for welding of 25 mm thick plates in flat position at a welding speed of 0.9 m min-1. For the obtain of full penetrated welds a laser beam power of 19 kW was needed. A high V-impact energy of up to 160 J could be achieved at the test temperature of 0 °C. Even at the most critical part in the weld root an impact energy of 60 J in average could be reached. The tensile strength of the weld reaches that of the base material. An introduce of the HLAW process with electromagnetic support of the melt pool in the industrial practice is an efficient alternative to the time- and cost-intensive arc-based multi-layer welding techniques which are established nowadays for joining of thick-walled constructions.
Im Beitrag sind die Vor- und Nachteile beim Einsatz von Ultraschallunterstützung für das Zerspanen von harten und spröden Werkstoffen zusammengefasst. Des Weiteren werden aktuelle und zukünftige Analysen zum Thema ultraschallunterstütztes Zerspanen mit geometrisch bestimmter Schneide schwer zerspanbarer Materialien und Rührreibschweißen an CNC-Fräsmaschinen vorgestellt.
TIG welding process was monitored using LIBS for the in situ measurement of chemical compositions in austenitic stainless steels. This research aims to prototype a real-time chemical composition analysis system for welding applications and prove the feasibility of such a quality control loop.
The chemical compositions of the weld pool, considering the welding metallurgy, is the most critical parameter for any occurring weld defects, e.g. hot cracking. Hence, controlling the weld pool chemical composition allows governing of the weld pool solidification behavior by monitoring and adjusting the respective welding parameters, e.g. welding current.
LIBS measurements were conducted during a TIG-welding process. The effect of the welding plasma on the LIBS signal was thoroughly investigated by varying various LIBS settings, e.g. delay and exposure time. Quantification of the main alloying elements Cr and Ni in the weld pool during welding was achieved by univariate calibration procedure.
A numerical framework for simulation of the steady-state thermal behaviour in keyhole mode welding has been developed. It is based on the equivalent heat source concept and consists of two parts: computational thermo-fluid dynamics and heat conduction. The solution of the thermo-fluid dynamics problem by the finite element method for a bounded domain results in a weld pool interface geometry being the input data for a subsequent heat conduction problem solved for a workpiece by a proposed boundary element method. The main physical phenomena, such as keyhole shape, thermo-capillary and natural convection and temperature-dependent material properties are taken into consideration. The developed technique is applied to complete-penetration keyhole laser beam welding of a 15 mm thick low-alloyed steel plate at a welding speed of 33 mm/s and a laser power of 18 kW. The fluid flow of the molten metal has a strong influence on the weld pool geometry. The thermo-capillary convection is responsible for an increase of the weld pool size near the plate surfaces and a bulge formation near the plate middle plane. The numerical and experimental molten pools, cross-sectional weld dimensions and thermal cycles of the heat affected zone are in close agreement.
Advantages such as element homogenization and grain refinement can be realized by introducing electromagnetic stirring into laser beam welding. However, the involved weld pool behavior and its direct role on determining the final microstructure have not been revealed quantitatively. In this paper, a 3D transient heat transfer and fluid flow model coupled with element transport and magnetic induction is developed for wire feed laser beam welding with electromagnetic stirring. The magnetohydrodynamics, temperature profile, velocity field, keyhole evolution and element distribution are calculated and analyzed. The model is well tested against the experimental results. It is suggested that a significant electromagnetic stirring can be produced in the weld pool by the induced Lorentz force under suitable electromagnetic parameters, and it shows important influences on the thermal fluid flow and the solidification parameter. The forward and downward flow along the longitudinal plane of the weld pool is enhanced, which can bring the additional filler wire material to the root of the weld pool. The integrated thermal and mechanical impacts of electromagnetic stirring on grain refinement which is confirmed experimentally by electron backscatter diffraction analysis are decoupled using the calculated solidification parameters and a criterion of dendrite fragmentation.
Over the past years economic and environmental considerations have led to a markedly increased demand for efficiency and flexibility in petrochemical plants. The operational temperatures and pressures required today can only be achieved by using new creep-resistant grades of steel. The modified 13CrMoV9-10 vanadium steel shows a good resistance against creep and compressed hydrogen and has been in use for the construction of petrochemical reactors since the mid-1990s. Nevertheless, processing of this type of steel requires extreme care during the welding procedure. This is due to its low toughness and high strength in the welded state when not post weld heat treated combined with increased susceptibility to cracking during stress relaxation. Previous research into crack formation in creep-resistant steels has largely focused on thermal and metallurgical factors; however, little knowledge has been gathered regarding the influence of the welding procedure on crack formation during post weld heat treatment considering real-life manufacturing conditions. The influence of heat control on the mechanical properties has been investigated by simulating the welding and subsequent post weld heat treatment operations during the construction of petrochemical reactors using a special 3-D testing facility on the laboratory scale.
This work is subdivided in two parts. In part I of this study the stresses resulting from preheating, welding, dehydrogenation heat treatment and the final post weld heat treatment were analyzed during experiments under varied heat control. In all experiments stress relief cracks formed during post weld heat treatment could be observed. The total crack lengths correlated with the welding induced stresses.
Part II of this work is dedicated to the characterization of the cracks and the microstructure. The application of a special acoustic emission analysis indicated that the cracks formed in a temperature range between 300 °C and 500 °C during the post weld heat treatment. In comparison to small scale specimens welded without additional shrinkage restraint, the toughness of the restrained welds was significantly decreased. SEM and TEM analyses of all samples revealed accelerated aging due to early precipitation of special carbides during post weld heat treatment under component relevant restraint.
A discussion is provided on demands that must be met in order to apply fracture mechanics to the determination of overall fatigue lifetime and strength, i.e., S-N curves and fatigue limits. These comprise the determination of the cyclic crack driving force for all stages of fatigue crack propagation, in particular for the short crack stage where the crack driving force has to be determined for elastic-plastic deformation and the gradual build-up of the crack closure phenomenon. Special emphasis is put on a fatigue damage relevant specification of the initial crack size. Different approaches in the literature are discussed. Another important aspect is the adequate treatment of multiple crack propagation. Finally, the discussion is illustrated by an example of a butt weld made of a medium strength steel.
Dissimilar metal weld overlays of nickel-base alloys on low-alloy steel components are commonly used in the oil and gas, petrochemical, and power generation industries to provide corrosion and oxidation resistance in a wide range of service Environments and temperatures. Traditionally, dissimilar weld overlays are produced using cold or hot wire gas tungsten arc welding.
This study aims to identify and evaluate potential advantages of low heat input gas metal arc welding processes over the conventional gas tungsten arc welding in the production of such overlays. Parts I and II of this publication series described characteristics of the heat-affected zone and the transition zone region of alloy 625 on grade 22 steel overlays. These results indicate a good resistance against hydrogen-assisted cracking, which is being verified within this third part of the publication series. To determine the hydrogen-assisted cracking susceptibility, welded samples are tested using the delayed hydrogen-assisted cracking test. Fractography is performed using scanning electron microscopy along with energy dispersive spectroscopy. The results confirm the suitability and efficiency of low heat input gas metal arc welding for dissimilar weld overlays. Variation of the postweld heat treatment procedure bears potential for improvement in this respect.
The analytical potential of a nanosecond laser ablation coupled plasma mass spectrometer (ns-LA-ICP-SFMS)system is investigated for fast and highly spatially resolved (~µm) elemental distribution within single cells. The size, morphology and overlapping of laser-induced craters has been investigated with Atomic Force Microscopy (AFM).
The sales of niobium carbide (NbC) have grown in the last years, but NbC is still a hidden carbide and mainly used as grain growth inhibitor in hard metals. In the present work it was shown that the progress in the key properties, like HV30, KIC and strength, followed by machining and tribological results of the respective NbC grades.
Das Schweißen hochfester Feinkornbaustähle mit modifizierten Sprühlichtbögen bietet die Möglichkeit, kleinere Nahtöffnungswinkel zu schweißen.
Allerdings findet der modifizierte Sprühlichtbogen in den Regelwerken zur schweißtechnischen Verarbeitung hochfester Feinkornbaustähle unter Vermeidung von Kaltrissbildung bisher keine Berücksichtigung. Dieser Beitrag betrachtet die Wechselwirkungen der Haupteinflussgrößen auf die Kaltrissbildung beim Schweißen mit kleineren Nahtöffnungswinkeln am Beispiel des hochfesten Stahls S960QL.
Effect of weld penetration depth on hydrogen-assisted cracking of high-strength structural steels
(2019)
The need for steels with highest mechanical properties is a result of the increasing demands for energy and resource efficiency. In this context, high-strength structural (HSS) steels are used in machine, steel and crane construction with yield strength up to 960 MPa. However, welding of HSS steels requires profound knowledge of three factors in terms of avoidance of hydrogen-assisted cracking (HAC): the interaction of (1) microstructure, (2) local stress/strain and (3) local hydrogen concentration. In addition to the three main factors, the used weld-arc process is also important for the performance of the welded joint, especially when using modern arc variants. In the past, the conventional transitional arc process (Conv. A) was mainly used for welding of HSS grades. In the past decade, the so-called modified spray arc process (Mod. SA) has been used increasingly for welding production. This modified process enables reduced seam opening angles with increased deposition rates compared to the conventional process. Economic benefits of using this arc type are: a reduced number of necessary weld beads and a lower weld seam volume, which result in decreased total welding time and costs. Nonetheless, investigations on a high-strength S960QL showed significantly higher hydrogen concentrations in the weld metal at a reduced seam opening angle with Mod. SA. This indicates an increased susceptibility of the welded component to HAC. Hence, existing recommendations on HAC-avoidance cannot be transferred directly to the Mod. SA-process. In the present study, the susceptibility to HAC of the HSS steel S960QL with same type of filler material was investigated. For that purpose, both Conv. A and Mod. SA were used with same heat input at different deposition rates. For assessment of the HAC susceptibility, the externally loaded implant test was used. Both conducted test series with Conv. A and Mod. SA showed similar crack critical stress of about 280 MPa. Below this value, no delayed fracture appeared. The welds with Mod. SA showed higher hydrogen concentrations. The fracture occurred in the heat-affected zone (HAZ) or in the weld metal (WM). However, the test series with Mod. SA showed a significant extension of the time to failure of several hours compared to tests carried out with Conv. A.
A numerical framework simulation of the steady-state thermal behaviour in keyhole mode welding has been developed. It is based on the equivalent heat source concept and consists of two parts: computational thermo-fluid dynamics and heat conduction. The solution of the thermo-fluid dynamics problem by the finite element method for a bounded domain results in a weld pool interface geometry being the input data for a subsequent heat conduction problem solved for a workpiece by proposed boundary element method. The main physical phenomena, such as keyhole shape, thermo-capillary and natural convection and temperaturedependent material properties are taken into consideration. The developed technique is applied to complete-penetration keyhole laser beam welding of a 15 mm thick low-alloyed steel plate at a welding speed of 33 mm/s and a laser power of 18 kW. The fluid flow of the molten metal has a strong influence on the weld pool geometry. The thermo-capillary convection is responsible for an increase of the weld pool size near the plate surfaces and a bulge formation near the plate middle plane. The numerical and experimental molten pools, cross-sectional weld dimensions and thermal cycles of the heat affected zone are in close agreement.
Class II hydrophobins are amphiphilic proteins produced by filamentous fungi with the tendency to accumulate at the air-water interface. They show unusual adsorption kinetics with a coverage-independent adsorption rate, which was monitored with ellipsometry and atomic force microscopy. A theoretical model was introduced to identify the underlying mechanism.
9 %-Cr steel P91 is widely used in power plants due to the excellent creep-resistance. Components of this steel are typically welded and demand for careful welding fabrication, whereas a so-called post weld heat treatment (PWHT), must be conducted to increase the toughness and decrease the hardness of the martensitic as-welded (AW) microstructure. Before the PWHT, a hydrogen removal (or dehydrogenation) heat treatment is necessary as hardened AW martensitic microstructure is generally prone to delayed hydrogen assisted cracking (HAC). The microstructure and temperature dependent hydrogen diffusion is an important issue as it determines how long a potential crack-critical hydrogen concentration could remain in the microstructure. In this context, reliable hydrogen diffusion coefficients of P91 weld metal are rare. Hence, the diffusion behavior of P91 multi-layer weld metal was investigated in two different microstructure conditions: AW and further PWHT (760 °C for 4 h). Two different experimental techniques were used to cover a wide range of hydrogen diffusion temperatures: the electrochemical permeation technique (PT) at room temperature and the carrier gas hot extraction (CGHE) for a temperature range from 100 to 400 °C. From both techniques typical hydrogen diffusion coefficients were calculated and the corresponding hydrogen concentration was measured. It was ascertained that both heat treatment conditions show significant differences in hydrogen diffusivity. The biggest deviations were identified for room temperature. In this case, the AW condition shows significant hydrogen trapping and up to seven times lower diffusion coefficients. Additionally, PT investigations showed a preferred diffusion direction of hydrogen in the weld metal expressed by the diffusion coefficients and the permeability for both heat treatment conditions. The CGHE generally revealed lower diffusion coefficients for the AW microstructure up to 200 °C. In addition, the AW condition showed hydrogen concentrations up to 50 ml/100 g (considering electrochemical charging). Nonetheless, this hydrogen was not permanently (reversibly) trapped. Nonetheless, this temperature is approximately 100 °C below recommended dehydrogenation heat treatment (DHT). This has two main consequences: (I) in case of welding is interrupted or no DHT is conducted, a HAC susceptibility of hardened martensitic P91 weld metal cannot be excluded and (II) DHT can be conducted at temperatures around 200 °C below the recommended temperatures.
9 %-Cr steels like P91 and P92 are widely used in power plants due to the excellent creep-resistance. Components of this steel are typically welded and demand for careful welding fabrication, whereas a so-called post weld heat treatment (PWHT), must be conducted to increase the toughness and decrease the hardness of the martensitic as-welded (AW) microstructure. Before the PWHT, a hydrogen removal (or dehydrogenation) heat treatment is necessary as hardened AW martensitic microstructure is generally prone to delayed hydrogen assisted cracking (HAC). The microstructure and temperature dependent hydrogen diffusion is an important issue as it determines how long a potential crack-critical hydrogen concentration could remain in the microstructure. In this context, reliable hydrogen diffusion coefficients of P91 and P92 weld metal are rare.
Hydrogen was once called “the versatile embrittler” [1], which summarizes very well the effect on reduction of ductility and/or toughness in technical alloys like steel. In that connection, welding is one of the most important component fabrication technologies. During welding, hydrogen can be transferred to the weld pool from manifold sources (like contaminations, residuals at the surface, etc.). As hydrogen embrittles a material, the safety of welded components with hydrogen is always a critical issue. Weld heat input causes additional changes in the microstructure like grain growth or partial dissolution of precipitates and many more. All these things influence the mechanical properties and also represent hydrogen traps. These traps decrease the hydrogen diffusion compared to the ideal lattice. The result can be so-called delayed hydrogen assisted cracking (HAC) of the weld joint due to the significantly decreased diffusivity by trapped hydrogen. This is often an underestimated risk as those cracks can appear in the weld joint even after some days!
It is essential to know about hydrogen ingress during welding and the microstructure specific hydrogen diffusion. Both are depended on weld parameter influence and the chemical composition of the base material and weld metal. For that purpose, gas analytic methods like solid-state carrier gas hot extraction (CGHE) are useful tools to: (1) identify detrimental hydrogen concentrations from weld joints, (2) binding energies from hydrogen traps by thermal desorption analysis or (3) high-temperature diffusion coefficients. Those values are extremely important for welding practice in terms of recommendations on realistic hydrogen removal heat treatment (HRHT) after welding. Considering the increasing use of “digital” experiments, the data is also needed for reliable numerical simulations of HAC process or HRHT-effectiveness.
The present contribution gives an overview on the influence of hydrogen on weld joints, the necessity, methods and standards for hydrogen determination (CGHE) with the aim of fabrication of safe welded and crack-free components.
[1] R. A. Oriani (1987), Corrosion 43(7):390-397. doi: 10.5006/1.3583875